I just finished the first volume of William Patterson's massive two-volume biography, Robert A. Heinlein: In Dialogue with his Century. It suffers a bit from the curse of too much information; e.g., "Shortly after selling his second story to Astounding, Heinlein feasted on a breakfast of bacon and scrambled eggs. The bacon was crispy, but the eggs were too runny. There was no toast." OK, I just made that up, but you get the picture. I'm being a bit unfair, as the most interesting thing about this book is the detailed picture it paints of pre-WW2 American society, especially Heinlein's life at the Naval Academy and his early career in the U.S. Navy. Like many science fiction readers, I had always thought of Heinlein as a writer with a brief and unimportant stint in the Navy. But Heinlein had planned on a life-long naval career before being forced to retire for health reasons -- I am sure that in a parallel universe somewhere he led the Pacific Fleet to victory against the Japanese.
Patterson also highlights another striking aspect of Heinlein's life -- his tendency to fall for many of the more unusual intellectual fads of his day. While he often presented himself as a hard-bitten empiricist, Heinlein latched onto some of the most bizarre variations of socialism in the 1930s, and he was a big fan of general semantics. What's that? You've never heard of general semantics? But it was all the rage -- or at least it was 80 years ago. Promulgated by "Count" Korzybski, it promoted "non-Aristotelian logic." As far as I can see, its major contribution to Western thought was its use as a basis for A.E. van Vogt's novel, The World of Null-A.
Of course, the 20th century is littered with the corpses of many other intellectual fads. One of my colleagues recently noted that the only people who believe in Freudian psychology anymore are English professors! This leads to an obvious question: which of our current intellectual fashions will disappear by the end of the century? Take your pick.
Patterson's biography has very little to say about the content of Heinlein's fiction -- if you're looking for that, I'd recommend Heinlein in Dimension by Alexei Panshin.