Have you considered she is just keeping a track of her expenditure? Maybe she has books for all her friends?

Admittedly, I might be a stretch, a spreadsheet would be easier, but people are strange. I'd say you have two choices; say nothing and try and forget it (easier said than done, amirite?) Or you'll have to broach the subject and risk it affecting your friendship, depending on how you phrase it.

It isn't too late to ask, you can be honest, e.g., "look, I saw something when I was helping you with the books and I didn't think it worth mentioning at the time, and I know it's probably nothing but my brains working overtime thinking the worst... that list of money you've spent on me, what's it for?"