English summary: The works of John Reginald Reuel Tolkien (1892-1973), author of Lord of the Rings, Silmarillion, and The Hobbit, have accounted for some of the most successful bookstore sales of the last half-century. But such popularity has its price: Tolkien is still a relatively littel-known author, unacknowledged for what he truly is, one of the greatest writers of our time. The author of Les univers d'un magicien would like to give readers of Tolkien's work a few keys to unlock the mystery of his art.This book first examines Tolkien's life and intellectual and spiritual origins: he was a reactionary anarchist and monarchist, a fervent Catholic in an Anglican England who was out-of-place in this century, fascinated by Scandinavian mythology and traditional thought, and frightened by technology and mass civilization. Tolkien vilified progress and argued in defense of a rural England, rooted in its History. To illustrate his point of view, he created parallel worlds: the Hobbits' world, for example, with its Elfs and peace-minded, feudal organization opposed to the forces of Evil, accompanied by their bestiary. Hundreds of characters are presented in his work, in which he brings several worlds to life. Tolkien is said to have created one of the most extraordinary and captivating complete worlds in literature, inspired by his pessimistic vision of humanity's future. It is all the complexity of this individual, transformed by his exceptional imagination and creative power, which this book attempts to explore. French description: John Ronald Reuel Tolkien est devenu un des ecrivains les plus importants de notre temps. Auteur du livre du siecle en Grande-Bretagne, traduit dans des dizaines de langues, lu depuis quarante ans par des millions de lecteurs, Tolkien est devenu un phenomene culturel et cela d'une maniere d'autant plus etrange qu'il ne cesse dans ses livres de celebrer un monde oublie, un monde fait d'imaginaire et d'heroisme chevaleresque, de nostalgie et de reverie, de fantaisie et de cosmogonie. Ce monde issu de la prehistoire, de l'age d'or et du Moyen age est devenu la reference de la grande litterature d'evasion, de la litterature de l'extraordinaire.Tolkien, les univers d'un magicien retrace et decrit les cadres et les structures, les societes et les communautes, les projets et les grands moments de l'oeuvre de Tolkien, telle qu'elle se laisse lire dans son infinie et inepuisable diversite. Il reproduit aussi la tension permanente qui sous-tend le texte dans ses grandes lignes, texte hante par l'idee du Mal et de la Subversion qui se manifestent tout au long de l'oeuvre la plus hallucinante de ce siecle.