InterWorld is just awesome. It fits in a very soft spot for me, resonating with quite a few of my favorite works plus a good number of my own works. It's something of the Heinlein Juvenile that Heinlein might have written alongside his "World Without End Quadrology". It's almost, to forgive an awful metaphor, what one might expect from a Harry Potter and the Princes of Amber: The Television Show, albeit without being a mutant hybrid [1]. It's the pilot episodes to the animated series of a young man discovering that he can walk through worlds, eventually signing up for an Army of alternate versions of himself. Small echoes there of The Man Who Folded Himself with some of the pseudo-dualism of The Longest Journey thrown in. The most difficult thing to the novel is that I at least would like to see it picked up for a few more seasons [2]. A simple, fun "juvenile" adventure story with a twist of multiversal meta-physics.

[1]Even if the successful Harry Potter and the Balance of Earth showed that such a crazy hybrid novel could work. It certainly taught me the benefit of protecting the planet from global warming and dark wizards. ("Sure, blame the dark wizards!")
[2]This novel actually is the result of the pitch of an animated series to DreamWorks that got shuffled around for a while. Nothing came out of it, balls were dropped, just like the television industry is known for. Now that the book is published, Neil Gaiman said that DreamWorks Animation has shown interest in this "new novel"... Heh.