The Thin Man is NOT Nick Charles (or William Powell). The Thin Man in the book is the missing professor, who apparently was terribly thin. The movies, however, were perfectly cast with William Powell and the incomparable Myrna Loy.
This was Hammett's last completed novel, and the first that he infuses with snappy, bantering humor. Nora Charles was supposed to have been based on Lillian Hellman, who, while not my favorite writer, must have been quite witty. This book is also an ode to alcohol -- whatever the question, alcohol is the answer.
It IS a pretty good mystery novel, but the mystery part does take a back seat.