Good point about the spline edge this is a peeve of mine as well. I do like that most databases store book height which is really useful imo.

The introduction to the edition of The Leopard that we have remark that the author and his wife were used to stashing important papers--letters, high-denomination bills, what have you--in their many books. Some they then forgot and lost.

I did once find a $20 I had left in a book; movie tickets and a postcard are about all I can remember finding of someone else's leavings.

Did you fold the bills so that you could slip them down between the cover and the spine?

Yes, folded once between the cover and spine. My one consolation is that it was probably Jowett's translations of Plato -- on the possibility that that would have been less likely to go straight to trash at Goodwill.