In November 1936 a group of English writers was fighting the Spanish civil war from a university library. The 20-year-old poet John Cornford and his friends were holed up in Madrid's faculty of philosophy and letters, where they fired guns from dishevelled library armchairs and blockaded the windows against snipers with thick tomes of German philosophy. Between battles they raided the English literature shelf for volumes of Coleridge and Wordsworth. Cornford, who was between history degrees at Cambridge, was delighted to have exchanged one university for another. Fired with anti-fascist enthusiasm, he was finding the war "terribly exhilarating".

Cornford was one of thousands of English and Americans to take up arms on behalf of the Spanish republican government after it was ousted from power by a fascist military coup in July 1936. A striking number of these were writers or artists, and they form the cast of I Am Spain. It's well known that writers including Cornford, George Orwell, Stephen Spender, WH Auden, John Dos Passos and Ernest Hemingway went to war, but this is the first book to show clearly who was where when, moving through the war month by month. The results are fascinating. David Boyd Haycock is a skilful historian, providing analyses of the political and military machinations at the same time as he brings the battles to life. And he makes the most of his colourful cast, strewing the narrative with splendidly unlikely tales of quirky heroism. By the end of the book it feels as though almost every eccentric British intellectual found his or her way to Spain.

For a generation of writers, the appeal of the Spanish civil war lay in its clearly-drawn lines of right and wrong. "When the fighting broke out on 18 July," Orwell wrote in Homage to Catalonia, "it is probable that every anti-fascist in Europe felt a thrill of hope." This was a generation who had missed the great test of the first world war and was longing for a chance to prove its courage. And they were not just fighting fascism but defending Spain itself, a country they had celebrated in the 1920s for its earth and blood, beauty and romance. "Spain is the real old stuff," Hemingway wrote in 1923, applauding the sumptuous tragedy of the bullfights. "It's just like having a ringside seat at the war with nothing going to happen to you."

Now Spain's battles had travelled from the bullrings to the streets. Soon Hemingway and many of his contemporaries were in danger. Cornford's sojourn in the university ended abruptly. An explosion blew a three-foot hole in the wall, wounding him in the head. In hospital he remained stoical, writing that "no wars are nice, and even a revolutionary war is ugly enough" but that he still had plenty of strength left. A month later, he was dead, killed in a terribly mismanaged battle at Lopera.

After Cornford's death the writers in Spain retained a sense of purpose and exhilaration, but they could never be so innocently hopeful again. By structuring his book chronologically, Boyd Haycock illuminates the quickly shifting moods of the war, showing how different it was to arrive in Spain in one month or another. In 1937 it was becoming evident that the republican cause was weakened by corruption and ineptitude. Orwell, arriving in January 1937, was initially impressed by a genuine workers' democracy. However, he quickly became aware that infighting between communists, Trotskyists and anarchists was sabotaging the republican ideal. Hemingway, who spent much of 1937 in Spain, remained more vociferously committed to the republican cause. He was furious when Dos Passos gave up on the fighting after learning that a close friend had been mistakenly executed as a spy: "What's one man's life at a time like this?" But in The Fifth Column, the play that Hemingway wrote in Madrid as the shells crashed around him, he portrayed Spain as a nightmare in which innocent men were killed by mistake.

There is more to say, I think, about the bizarreness of Hemingway's play, written in the middle of a battle. What is missed in Boyd Haycock's account is the strangeness of writers and artists going to war. Many of the book's protagonists experienced the war partly as an imaginative event, whether it was Hemingway writing his play or Robert Capa staging his famous photograph Loyalist Militiaman at the Moment of Death (a picture which is not mentioned alongside Capa's appearances here). The role of the media in the Spanish civil war meant that more than ever before the artist-soldiers were caught between art and life. Because propaganda was crucial to securing funds for weapons from outside Spain, this was a war of words and images. Writers and artists were aware of the photographs they had seen and descriptions they had read, and of the need to add to these in their turn.

Although the book takes its title from a line in Auden's poem "Spain 1937", Auden and Spender are largely absent. Admittedly both did almost nothing to further the republican cause on the ground. But their poems and reports were fundamental not just as propaganda but as a demonstration of what made this war unique. In an essay about Picasso's Guernica written in 1938, Spender stated that Picasso's picture was not a response to personal horror but to "horror reported in the newspapers, of which he has read accounts and perhaps seen photographs". "This kind of second-hand experience," he adds, gleaned from the media, "is one of the dominating realities of our time." The poems, pictures and films to come out of the Spanish civil war are compelling because they are so often self-conscious; because they reflect the bizarre experience of the artist at war, observing at the same time as fighting, grappling with the conundrum of "I am Spain".

In one of the book's many stylish vignettes, Boyd Haycock describes the making of Joris Ivens and Hemingway's 1937 film The Spanish Earth. This is a film that is consistently aware of the unprecedented intimacy of its own war footage. "Man cannot act before the camera in the presence of death," Hemingway states in the commentary. Yet, granted ringside seats at a war they were fighting themselves, this is exactly what the cast of Boyd Haycock's book was doing.

 Lara Feigel's The Love-charm of Bombs: Restless Lives in the Second World War is published in January by Bloomsbury.