Phantastes

Phantastes: A Faerie Romance for Men and Women is a fantasy novel written by George MacDonald.
This the original and unabridged text formatted for modern readers.
The story centres on the character Anodos ("pathless" or "ascent" in Greek) who is a young man who is pulled into a dreamlike world and there hunts for his ideal of female beauty, embodied by the "Marble Lady". Anodos lives through many adventures and temptations while in the other world, until he is finally ready to give up his ideals.
C.S. Lewis wrote, concerning his first reading of Phantastes at age sixteen, "That night my imagination was, in a certain sense, baptized; the rest of me, not unnaturally, took longer. I had not the faintest notion what I had let myself in for by buying Phantastes."