As a kid? Around 10 years old my Father got me hooked on science fiction books starting with Larry Niven and moving on to Laumer, Chalker, Frank Herbert, and Heinlen.
My favorite books...
Really a tie between Ringworld by Larry Niven and the Bolo series by Keith Laumer.
Both had me teaching myself physics from my Father's college books in middle school(11-13 years old) in order to actually figure out what was going on... Best education a kid can possibly receive in my opinion. Hook 'em on science fiction then give them an algebra textbook and a physics textbook and let them learn it by themselves.
Best book as an adult? Pick a book written by Terry Pratchett, or any of the Sten series.