For sheer enjoyment (measured in how many times I've re-read them, at least at various times in my life): Heinlein's _The Door into Summer_. Connie Willis' _Bellwether_ (these can be funny, right?). Ayn Rand's _Atlas Shrugged_ (more in my youth than today, but I can probably still recite passages by heart).

Non-fiction: Gerry Weinberg's _Secrets of Consulting_. Lynn Truss' _Eats Shoots & Leaves_.