So where, one might ask, does one start with Robert A. Heinlein?
Well, frankly, you could start just about anywhere! Heinlein won the Hugo Award four times for best novel (for Double Star, Starship Troopers, Stranger in a Strange Land and The Moon is a Harsh Mistress, since you ask) during his lifetime and added another four posthumous Retro Hugos. He was one of the so-called Big Three of SF authors, along with Isaac Asimov and Arthur C. Clarke, and was often called ‘the Dean of Science Fiction’, so if you pick up one of his books at random, you’ve got a decent chance of reading something special.
However. Since one of the purposes of this site is to act as a guide through the classics of SF, it’s probably a bit of a dereliction of duty to leave it at that. So, if you press us, we’ll go out on a limb ( a pretty sturdy limb, of course) and say that you should start with one of his SF Masterworks: Double Star, The Door into Summer, Starship Troopers or The Moon is a Harsh Mistress . . .