After scanning my shelves it looks like the earliest Heinlein book I have handy is
Methusaleh's Children, so I've been reading that for the past hour or so while I've been scanning my computer for viruses. This is the first book of Heinlein's I've read which had a future history diagram included in it, which is fascinating. I never cease to be impressed by not only Heinlein's technological innovations (for instance the self-driving car in this one) but also the nonchalant way he works it into the story without stopping the plot place to digress into the tech side of it. Lazarus Long is a cool character who reminds me somewhat of the hero in
Puppet Masters. The social conflict between the normal humans and the long-lived humans seems to foreshadow X-Men in many ways.
I think after I finish this I want to read Sixth Column if I can find a copy. I'm trying to read Heinelin's books in approximate chronological order as much as possible. Will need to find some of his early stuff I don't have, though--after this and Puppet Masters the next thing I have is Revolt in 2100. Is there a good Heinlein anthology with some of his early short stories/juveniles collected?
There are three anthologies of Heinlein shorts:
The Green Hills of Earth, The Man Who Sold the Moon, and
The Menace From Earth. Each has some really excellent stories, and some less excellent.
Other than the shorts, I like the juveniles, Starship Troopers, Puppet Masters, and The Moon is a Harsh Mistress. I dislike the other later stuff.