When it comes to prose, our current book culture rates the sound as being less important than what the words say  content wins out against what we unfortunately term "style". But in former times we understood the subtle relationship between meaning and verbal music (or music itself, as in madrigals and part-songs). The straightforward, sonorous prose of Sir Thomas Malory's Le Morte d'Arthur, appearing from Caxton's revolutionary presses in 1485, lies somewhere between Middle English and modern: once its spelling is updated and a short glossary provided, it can be relished without difficulty.

Malory uses repetition to trance-like effect. The surface of the prose sculpts itself smoothly and beguilingly, eased by the little supple chain-links of "and", the weight always falling softly on the key word. The pace quickens for the gut-spilling action and slows for plangent reflection: "Wit you well then was there many bold knight there with King Arthur that wept as tenderly as though they had seen all their kin afore them." The long-vowelled build-up to the abruptness of "kin" is masterly  and "kin", along with "adventure", is the work's chief theme. Both are as costly as falling for the wrong person: this sentence vividly expresses the grief that follows the "mortal war" fought between Arthur and Sir Lancelot over Guinevere, a conflict spurred on by Arthur's illegitimate son Mordred.

The real difficulty is that, for all its weird and wonderful action, Le Morte d'Arthur is as muddled, ramblingly repetitive and inconsistent as a dream. Until the final four books (out of 21), it lacks any depth of characterisation, and the castles, forests and shorelines are indistinguishable. The action itself is often brutal and motiveless under the chivalric glitz and Christianised sheen: it includes murder, incest, rape (in two instances by a woman), madness (Sir Lancelot's, caused by said rape), mutilation (the swordfight in Monty Python's film version was no exaggeration) and massacre. Its Sicilian-style blood feuds involve fratricide, deliberate or accidental: early in the work, the two loving brothers Sir Balin and Sir Balan kill each other unknowingly. If Malory had allowed his knights to lift their visors and identify themselves before, instead of after, their mutual hackings, we would lose a good part of what slender plot there is. The long-suffering women are a relief  when they're not being beheaded.

The shadowy Sir Malory himself was typical of what Galahad calls, with masterly understatement, "this unstable world": a member of parliament, he was accused of burglary, rape, extortion, ambush and wounding  spending two periods in prison, in one of which he seems to have killed time by writing his masterwork. Steeped in the flouncing, fast-decaying world of joust and tournament, he probably saw the bloodier stuff abroad at the siege of Calais. His achievement was to stitch together the numerous French and English source tales into an uber-compilation that has held the imagination ever since, generating an Arthurian industry that reached its peak in late Victorian times (most memorably in Tennyson's "Idylls of the King") and which, from Disney to online gaming, shows no sign of fading in our own.

If this impact was achieved mainly through the work's simple, spellbinding language, any "translation" needs to be finely tuned. There have been numerous such attempts: Peter Ackroyd's The Death of King Arthur is one that's self-confessedly "loose" (but confusingly billed on the title-page as a "retelling"  was the editor in?). This is unfortunate: while the heavy cutting and light restructuring are acceptable, the deadpan delivery and novelistic additions ("he smashed his fist against one of the tapestries that lined the wall of the great palace") are less so.

During one of the final battles, Malory recounts how "by evensong time, Sir Launcelot and his party better stood, for their horses went in blood past the fetlocks, there was so much people slain". Ackroyd's version retains the fetlocks  a rare detail in Malory  but discards the lyrical counterbalance of "evensong time": "During the course of that day Lancelot's party got the better of the engagement; the very fetlocks of the horses were covered in blood." This has about as much presence as a police report of a Friday night in Cirencester, and doesn't mean quite the same thing: Malory suggests they were doing better because there was so much blood. The hinge word is "for", which Ackroyd cuts. But masterpieces are all in the detail. At times Ackroyd achieves his own poetry. When the dying Arthur suspects Sir Bedivere has not, after all, cast Excalibur into the lake, Malory recounts it thus: "What saw thou there? said the king. Sir, he said, I saw nothing but the waters wap and waves wan." This is a beautiful, stumbling lie  "wap" meaning "ripple", and "wan" echoing the frequent appearance of "wanhope", meaning despair. But Ackroyd's wholesale revamp also works: "'So what did you see?' the king asked him again. 'I saw nothing, sire. Just the long lake beneath the sky.'" Yet the overall effect is like visiting a smaller, concrete version of King's College Chapel  without the choir.

Adam Thorpe's novels include Hodd (Vintage).