The self-made man, working his lonely way by night through the best the world's thinkers had to offer, was an ­attractive cultural icon of the early 20th century. The masses' thirst for betterment spurred intellectual popularisers such as HG Wells. Voracious readers from the lower orders spent their pennies not only on primers and surveys but on series designed for them, such as Dent's Everyman's Library or Grant Richards' World's Classics, that offered the original texts themselves in cheap editions. Nor, of course, was this thirst for knowledge and self-improvement an exclusively British phenomenon. As is readily apparent from Timothy Ryback's thoughtful and oddly intimate book, it existed in central Europe, too. Anyone familiar with the obsessive drone of his wartime Table Talk or Mein Kampf will have already suspected what is amply demonstrated here: Adolf Hitler was the ultimate autodidact.

Hitler's Private Library begins with the striking image of the 26-year-old Hitler, serving at the front in northern France, walking into the nearest town during a lull in the fighting  to buy a book. How did the young serviceman plan to pass the time, when not dodging enemy fire delivering messages to the front? By settling down with Max Osborn's popular architectural history of Berlin. Three decades later, he and his court architect, Albert Speer, would relax during the second world war by poring over scale models of the Berlin of the future; but in 1915 he was already a keen devotee of the subject. The volume that the young soldier bought in Fournes survived both wars: and it is just one of the items intelligently discussed by Ryback both as object and as guide to its owner's interests and obsessions.

Books surrounded Hitler his entire adult life  though whether they opened his mind and formed his worldview or simply bolstered opinions that had already germinated is open to doubt. What is not in question is their importance to him, evident long before the party hacks sent in their endless complimentary volumes to clutter the shelves in his private residence. Books had travelled with him from the outset of his political career. Indeed, in the modest Munich apartment he rented after resigning his army commission, his first piece of furniture was a wooden bookcase, quickly crammed with cheap detective novels, military histories and memoirs; on its shelves, Montesquieu, Rousseau and Kant rubbed shoulders with anti-semitic philosophers, nationalists and conspiracy theorists. The future Führer was improving himself for all he was worth, for in Kultur-proud Weimar Germany even a rightwing revolutionary had to overcome the deficiencies of an abbreviated education if his poor grammar, bad spelling and other howlers were not to cost him the command of the beer-hall battalions and hand over leadership of the growing Nazi movement to his intellectual superiors.

In the 1930s, as his political fortunes improved spectacularly, and with them his income from the sales of Mein Kampf, Hitler spent more and more on books until they became one of his largest items of expenditure. By the mid-1930s, he had thousands. And the nightly read continued to the very end: piles of books crowded even his spartan last bedroom in the Berlin bunker. In the photos taken by US servicemen they can be seen by the bed where he finished up, slumped in a pool of blood next to the corpse of Eva Braun-Hitler.

Ryback is not primarily interested in itemising the works in Hitler's possession; others have done that and in doing so missed what he offers  a careful perusal of individual volumes that were either marked up by their owner or personalised with dedications offered to him by their authors. In terms of content, there are few surprises: Hitler's pencil was drawn to pronouncements on the nature of genius and the importance of leadership; the books that languished unread included a biography of Gandhi and a massive collection of the latter's prison writings. Many passages provoked him to violent scrawls of outrage. But both agreement and disagreement prompted the pencil. We almost catch the man in the act after he has retired to bed for the night, his reading glasses, book and pot of tea already laid out for him by the servants. It is easy to laugh at all of this, to mock Hitler for his pedantry and his ignorance. But his accumulated knowledge had real-life consequences. Military matters, for instance, dominated in his private library, and his keen reading of the biographies of great generals and contemporary almanacs of military equipment helped to give him a confidence by 1939 that he had lacked 20 years earlier. Now, he was not afraid to stand up to his own advisers, and the result was a war that tore Europe apart.

Hitler's friends from his early years always associated him with books and stressed that it was a "deadly serious business" for him. One thinks of those heavy yet largely unread thinkers, those busts of Schopenhauer and Nietzsche that the Führer liked to surround himself with, the great encyclopedias that he consulted constantly. As an approach to turning the Führer back into a real human being, of his time and place, Ryback's strategy works rather well. Yet what we are left with is not the mysterious cipher that some of Hitler's recent biographers have offered us so much as a dull and slightly depressing petit bourgeois with a chip on his shoulder.

Mark Mazower's books include Hitler's Empire: Nazi Rule in Occupied Europe (Penguin).

Dictators tend to be night workers, immune to the exhaustion that topples the rest of us. Napoleon, in an official portrait by David, posed in a study with closed curtains and a clock marking 4am; as if on sentry duty, the vigilant emperor oversees his dormant, submissive realm. Margaret Thatcher boasted of making do with an hour or two of sleep. The sergeant who issued orders to Corporal Hitler in the trenches in 1915 was impressed by his insomniac underling, who even then seemed - at least in the officer's obsequious recollection - to be destined for greatness. When the sergeant stomped into the dormitory to find a message runner at 3am, Hitler always volunteered. "Let the others sleep," he would say with a martyred smirk. "It doesn't matter to me."

But such hyperactive despots have a problem. What can you do during those white nights, with the rest of your government peaceably snoring? Hitler spent his sleepless hours reading. In Landsberg Prison, where he was locked up after the Munich beer hall putsch in 1923, the fawning guards waived the electricity curfew so he could read until dawn. Even after his installation as chancellor of the Reich, he consumed one and sometimes two books a night and next morning summarised his intake of pettifogging data to his secretaries in what Timothy Ryback calls "extensive, often tedious detail".

Interruptions of these solitary sessions were not tolerated. Once, when Eva Braun disturbed him, he ejected her "with a tirade that sent her hurtling red-faced down the hallway". No comfort his mistress offered could rival what he absorbed from his handbooks on tanks and war fleets, his manuals illustrating racial typology of the German people. Hitler was not a passive, open-minded reader, anxious to spend time inside someone else's mind. He read, as Ryback puts it, "intensely, fiercely". In the redoubt near the Belgian border from which he directed military operations in 1940, a field marshal billeted in an adjoining bedroom could hear him turning pages through the thin walls.

This ranting monologuist did not defer to the writers of the books he took to bed with him. Ryback, examining volumes from Hitler's library that were crated up and taken from the Berlin bunker to the Library of Congress in Washington in 1945, is startled at first by the marginal annotations he finds. He wonders at the capacity of Hitler, who in conversation had little tolerance for the opinions of others, to "engage with the text". But are those jottings the result of engagement or of a peremptory conscription? The marks do not point to fine writing or single out ideas needing further thought. They are calligraphic calls for action.

Even Ryback is chilled by the pencilled emphases in Hitler's copy of an essay by the antisemitic nationalist Paul de Lagarde. Sometimes, the marks are "vertical strikes", like a diagram of bombing raids; a passage about the solemn responsibility of being German is framed by Hitler with "a phalanx of three dense lines to the left and three equally intense lines to the right", as if he were mobilising troops to appropriate the paragraph.

Fingering the often toxic volumes with their racist diatribes and their mad metaphysics, Ryback feels himself to be in the presence of their previous owner. To hold one of Hitler's books is the equivalent of agreeing to a handshake, and the act, as Ryback realises with a shudder, leaves physical traces behind. Inside Hitler's copy of an architectural guide to Berlin, bought during his military service in 1915, Ryback finds "a wiry inch-long black hair that appears to be from a moustache". At least a red drip on another page, still viscous after 90 years, turns out to be paraffin oil, not human blood. Milton venerated great books as "the precious lifeblood of a master spirit", but that vital stream can also transmit disease.

As a bibliophile, Ryback is reluctant to blame Hitler's books for their odious contents. Book-burning should be left to bigots; the liberal humanism despised by Hitler insists that his books must be preserved and treated with respect. But Ryback describes them with a nervous dread that often quickens into macabre poetry. Thus the stained book that contains the specimen of facial hair has "corners that curl inward like dried lemon rind" and a spine with "fraying linen tendons, exposing the thread-laced signatures like rows of rope-bound bones"; Ryback has described a cadaver.

If they're good enough, the books in Hitler's library have repudiated a reader who used them as evidence for his crazed creed. This, fortunately, is the case with Shakespeare, for whom Hitler had an embarrassing admiration. He often quoted Julius Caesar and cited it as a precedent, regarding the Ides of March as a day when fateful decisions should be made and threatening opponents, like Caesar's ghost, that he would see them again at Philippi. But the same play, when Orson Welles staged it in New York in 1937, became an attack on fascist regimentation, showing up Hitler's opportunistic misinterpretation.

Dictators, as Ryback demonstrates in his brilliantly conceived and meticulously researched study, can never monopolise the truth. To read, we need the sun or at least a lamp, as well as a mind that is awake rather than nodding off, as Hitler habitually did, into obscurantist fantasy.