Since I cannot reread Heinlein, I should not read a biography of Heinlein
But I can browse one. William H. Patterson's Robert A. Heinlein: In Dialogue with His Century, Learning Curve 1907-1948, would appear to be definitive. The very thick volume one — over six hundred pages with notes – stops at 1948. It is very well written and engaging and connects Heinlein to broader American history. There is plenty on Heinlein and free love, Heinlein and H.G. Wells, Heinlein in the Navy, Heinlein and Missouri, and many other topics.