"The Unpleasant Profession of Jonathan Hoag" is a short novel that was originally published in Unknown Worlds in 1942. It is most easily accessible, as far as I know, as the title story of Heinlein's collection, The Unpleasant Profession of Jonathan Hoag. This also includes such classic stories as "'- All You Zombies -'" and "'And He Built A Crooked House'" (in each of these cases, the title of the story includes inverted commas).
Although it's not of great interest to me for the purposes of my forthcoming book on science fiction and the moral imagination, "The Unpleasant Profession of Jonathan Hoag" is an amusing story that ultimately takes a dark - almost horrific - turn. It follows a husband-and-wife team of private investigators hired by the amnesiac Jonathan Hoag to discover what he actually does at work during the day (each evening, he forgets the events of the day; he lives a somewhat luxurious lifestyle without knowing where his comfortable salary is coming from).
Although the final pages are frightening, the explanation of Hoag's eponymous "profession" does not seem satisfactory or entirely coherent. There's a vibe here of an author making things up as he went along, though I don't know how carefully the book was planned or revised.
The bottom line for me is that this is a short novel that Heinlein aficianados and SF aficianados can do without. It is lighthearted, almost comic, fantasy that gradually morphs into something more like a horror story. But you can enjoy the ride. It's only a couple of hour of reading, and it rewards the effort.