This makes me feel not so bad missing that mate in two earlier this week:
Fischer's note with the puzzle's solution says he missed this posibility but won anyway. I Googled for the actual continuation but didn't find the game. What I did find was a game with a similar position and it looks more like Bobby missed a mate in 3.
41. Kf1 e4 42. Bf2+ Kh3 (Kh2 ends the same) 43. Rh7#
I had gone back to Bobby Fischer Teaches Chess because I was thinking it might be a good book to put in my daughter's hands (easily portable, has puzzles which she likes doing). Unfortunately, I used a pen to write the solutions for the first half of the book right on the diagrams.
Guess I'll have to buy it as a stocking stuffer for her.