My Mom used to hide money in books. Sometimes a kid of her's found it when she encouraged us to go read something, hahaha. So, I never hid it in books but once I hid a fifty under a carpet and later moved, a couple years later I woke with a sudden memory. Shit! Now I hide it in banks and investments. Mind you that isn't always safe either. I lost on an oil investment once, not much because I was just starting to play this way but it did hurt my feelings a little, now I can laugh a bit about it but it still sets off a little tingle in the back of my head. I have also lost money lending it to someone in need, have tightened those belt loops quite a bit since.
Says who?? He could have hidden it in 5s and 10s. Then again, it might be two 50s ... bigger still. (Note for American readers: in sensible countries like the UK, different denomination bank notes are different sizes ... and have different designs on them)
Rudimentary deduction,BeachBum-£100 in 5's is 20 sheets;enough to show-and likely cause the book to fall open. £10's...well,not so much,but still ten sheets of paper hidden between pages should show. FIVE sheets,however,may well be flat enough to remain concealed-especially if it's a hardback. And two £50's would quite probably slip and show,due to lack of cohesive friction. Y'see? Elementary,my dear Wotsit.