Sometimes things like this sneak up on you, but in this case I think I had no idea this was going on. Not that I could afford to attend, anyway, but it would have been nice to celebrate by re-reading some of my favorites.
The celebration is weekend-long in Kansas City, MO. Guest speakers include everyone from Arthur Clarke to John Scalzi.
Heinlein is one of the grand masters of Science Fiction, and in my list ranks somewhere below Asimov and above Pohl. I remember starting with "Tunnel in the Sky", then "Citizen of the Galaxy". Then being blown away by "Stranger in a Strange Land". You grok?
But where would one start today? If you're younger, we'll say pre-teen, then "Tunnel in the Sky" or "Citizen" are great books to get your hands on. I've likely re-read "Citizen" ten times, and it's not the science fiction that pulls me back - it's the characters and the story, again and again.
"Stranger in a Strange Land" brought Heinlein many new fans in the early sixties. Indeed, hardcore Heinlein - o - philes will tell you that there are two types of Heinlein fans: those that came in through SIASL and those who were there before. "Stranger" was written to break Heinlein out of the "children's author" pack, and break him out it did.
You can find a pretty pervasive site recapping Heinlein and his works here.