The hero of Doorways In The Sand has managed to stay in college for something like 14 years without graduating. He gets close at times, but always changes majors or otherwise manages to escape. The reason is that his uncle's estate pays for his college education until he graduates, but doesn't contain a time limit.
As books by Zelazy go, Doorways In The Sand is good, but not one of the greats. It's fluffier than something like Lord of Light, and lacks the grand sweep of the Amber Chronicles. Still, it's fun and well written. Definitely worth your time if you're into lighter SF.
After Lord of Light, this one (1976) is probably Zelazny's best-liked science fiction novel. The inventive plot involves a frozen uncle, realistic bad guys, and undercover alien cops. It has wacky inventions: dog suit so alien coppers won't be conspicuous on Earth; a Rhennius machine for reversing one's self and others; and a star stone, the function of which cannot be revealed in a review. The likeable hero Fred Cassidy is an eternal student (a wise-acre polymath, similarly to other Zelaznian protagonists) and a parkour artist before parkour was cool. Zelazny's ideas were copious and odd, but, in the context of the universes in the stories, also consistent and reasonable.
Interesting and action packed, aliens and surprise twists.