What you are missing is in the combination that follows. The queen needs to be on the f1-a6 diagonal to make it work. Black takes the rook on f1 which leads to mate if black recaptures (bishop, then rook).
Play through the whole series starting with Qa6, then you will see how it would not be possible to win that amount of material from Qa7.
http://www.chess.com/chessmentor/view_lesson?id=13
Wouldn't Qa7 win the pawn? And, once the pawn is won, an attacking diagonal is opened up for the queen, just as in the book solution.
What am I missing?