I read that book years ago (back in the days - our undergraduate days - when, apart from J. R. R. Tolkien, my alienated intellectual male friends read T. H. White; fantasy, world creating, and/or alternative history literature was really rather limited then. It experienced an explosive growth later).
On their recommendation, I read the lot - Tolkien and T. H. White - and remember that the early parts of the book are brilliant, beautifully written and really clever, but I seem to recall that it sort of runs out of steam a little towards the end.