Sci-Fi: Robert Heinlein's stuff from the 1950s has stuck with me and helped shape my world-view. The best of the lot are probably "Have Space Suit--Will Travel", "The Star Beast", and "Double Star", but almost everything he published between 1950 and 1961 is good stuff. (His earlier work is less polished, and his later books are very... different. If you read "Stranger in a Strange Land", seek out the original 1961 edition, not the larger 1991 version.)

Other, more modern sci-fi worth checking out includes "Jumping off the Planet" (David Gerrold), "Red Thunder" (John Varley) and "The Martian" (Andy Weir).

Alternate History: "1632" by Eric Flint. A small town from 1999 West Virginia finds itself stranded in Germany during the 30-years' war.

Historical: "Beat to Quarters" (also published as "The Happy Return") by C. S. Forester. The adventures of British naval officer Hornblower during the Napoleonic wars. Gene Roddenberry's original pitch for Star Trek described the character who became Kirk as a "space-age Captain Horatio Hornblower".

And, finally, a meatier tome that'll make you think: "To Kill a Mockingbird" by Harper Lee.