This is a puzzle in a book I'm working through.  The book says Re2 is the answer, and only gives the first move.  I thought this was the answer, but the answer guide always only gives 1 answer.  Can anyone see anything wrong with my answer?

I think this puzzle has two answers. though your book presented only one of them.

The computer confirms it: your line is also a mate in two. Therefore, the puzzle is borked.

The king doesn't have to take the nf4, therefore when bishop checks he can still move to f2 

... you did see the alternate lines, right?

i did not... 

I think with 5334 puzzles and whatever, it's reasonable to only put 1 answer.  There are some others I saw that I think also have multiple answers.

That explains it.  If the king doesn't take the knight, the knight moves again to check the king, giving mate.