Perhaps the most famous tale of a book's origins is that of the ghost-story competition between Lord Byron, the Shelleys and John Polidori at Montalègre in Switzerland, which brought about the birth of Frankenstein. If Brian Aldiss is to be believed, this was also the birth of science fiction.

The Lord of the Rings does not represent the birth of fantasy, but certainly represents the birth of genre fantasy. And, while the Inklings did not in the same way cause LOTR to be born, they at least assisted at the delivery — if only (as most men in western films do) by boiling plenty of hot water (in this case, making it into tea).

The Inklings were an informal literary group, one of many that existed at Oxford University in the 1930s, 40s and 50s. They discussed literature and philosophy, and read and criticised each others' literary efforts. Their most famous members were the fantasists J. R. R. Tolkien, C. S. Lewis and Charles Williams.

This book, first published in 1978 (but newly released in paperback this year) is written by Humphrey Carpenter, author of J. R. R. Tolkien — A Biography. Perhaps for that reason, The Inklings concentrates on the lives of Lewis and Williams, and gives Tolkien the walk-on parts. However, this is not just a companion volume, but a tale in its own right, and I soon forgot all about the bias — partly because it makes sense to approach the Inklings through the eyes of C. S. Lewis, who brought them all together in the first place, and partly because it's so well written.

Lewis placed a strong emphasis on friendship. At the time he met Tolkien, he was at a turning point in his life, being just about to take the plunge into Christianity. Tolkien's already-formed ideas about the relevance of myth to daily life allowed Lewis to understand his chosen religion with the logic and rationalism that had at first destroyed his belief in it. The two men found they had much in common, including a love of old literature, Norse myths, and a dislike of modern fiction. This common ground allowed them to appreciate and criticise each other's creative efforts, and they became fast friends.

Then Charles Williams appeared on the scene. Where Tolkien and Lewis had held a quiet belief in their ideas about the relevance and "truth" of myths, Williams was a sort of embodiment of those ideas — he lived them. He had dabbled in the occult as a member of the Golden Dawn, wrote complex (and increasingly impenetrable) poetry, lectured on literature in his spare time (while working as an editor for the Oxford University Press), and had gathered a coterie of followers, who were semi-religiously drawn to his enthusiasm and mystical philosophy. Lewis took to him, and even described him as an "angel". Tolkien disliked him.

Although it was after Williams's death that the Inklings fell apart, his coming marked the first (subtle) distancing between Lewis and Tolkien, whose friendship finally disintegrated over a combination of things — Tolkien's dislike of The Lion, The Witch and the Wardrobe, Lewis's criticisms of Tolkien's Catholicism, and, finally Lewis's marriage to Joy Davidman (perhaps because it was hypocritical of him to expect the Inklings to accept Joy as one of their number, when he had never taken an interest in their domestic lives).

The story of the writing of the classics of modern fantasy these three men produced (The Hobbit and The Lord of the Rings, the Interplanetary Trilogy and the Narnia books, and, by Williams, such occult thrillers as War in Heaven and The Place of the Lion) is incidental to their growing and fading friendships.

Humphrey Carpenter attacks the subject in a lively and varied manner. He does not merely start at the beginning and list his way through the facts. He focuses on the character and views of the members, discusses the question of whether the Inklings can be considered a literary "movement", and even presents a fictionalised sample meeting, where the members' words are drawn from their work and writings. Carpenter actually builds the story into a drama. This is the sort of thing that biography should be about.