Dying Inside by Robert Silverberg
The Book of Skulls by Robert Silverberg
I could make a long list of great sci fi / fantasy. But these two IMO are quite readable as an adult at any age. More age durable in a manner of speaking.
A separate note. If you keep in mind that it was written in 1963 (relevant for several reasons), Glory Road by Robert A. Heinlein is a lot of fun. Sort of swashbuckling without the sex and profanity.
Glory Road would make an awesome movie.