Ahh yes, Heinlein, one of my all-time faves. I read many of the "juveniles" growing up (in Hickory, NC, thank you, Tonchi!
) including Between Planets, Time for the Stars, The Rolling Stones and Space Cadet. I also mixed in some stuff from the grownup section of the library like Double Star and The Door Into Summer. But the all-time favorite was "Have Space Suit, Will Travel." Unless some soulless bureaucrat threw it out, somewhere there is a Scribner's hardback of this book from my elementary school's library with my name signed on the card at least a dozen times. (The school was closed down for years, and burned down earlier this year. I like to think if they didn't keep it for some other school library that it found its way to some other appreciative youngster.)
In the more mature phase, I'd have to hold up The Moon is a Harsh Mistress and The Man Who Sold the Moon--and for that matter many of the short works of the Future History universe. I Will Fear No Evil was a racy treat for the inexperienced palette of a young teenager, while Stranger in a Strange Land was grist for someone becoming skeptical of religion. Friday was perhaps the best of his late phase, Job was not without charm. The Number of the Beast was a fascinating conceit. I confess I never really got the point of The Cat Who Walks Through Walls, and I couldn't put To Sail Beyond The Sunset down even though I very, very badly wanted to.