How many should you donate?

Depends on what you want from the shop. For instance a lot of people are shooting for 400 which is the price of the malice which is nessacary for an acheivement.

So you may wanna just skim the shop and if there is anything you want go for its price/points

Or if you are into getting acheivements scope out the acheivements associated with donating books and head for those as well XD

Donate as many as you can! (: I think the best way to get the cheaper books is to try rs-ing some at the Book Nook, then maybe buy what else you can afford.

Even though you can't buy from the shop yet, you can still go in and look at the stuff. See what you want to buy, then donate based on that total. But remember, they may update the items, so I would donate a few extra, just in case.