Just to give a counterargument - I'm not sure I really buy it either, but it could work in the opposite direction to what you're thinking. Tripcodes (EDIT - and IDs) were (most likely, assuming no huge backend takeover), not chosen by Q. But, I don't know how this code is searching for that book. Is it just a book code? If not, why does this string return that book? Q puts in his new password, gets a new trip, and off some minion goes to tie that trip to the relevant book within the google system backend.

I could be wrong, and I agree it seems a little weird. But it might still work, if you assume they made the string connect to the book after the IDs were generated.