If a member needs urgently a particular book he could pay few bucks (five euros?) for a very short access to donator zone. I can't see any disadvantage in this method and an income will be guaranteed to cover the forum expenses.
It could be possible also to separate members who want to donate to support the forum and members who want specifically contribute to buy a book. As I said in a previous thread there is a way to buy multiple books using the same money.
Maybe a survey could be useful to evaluate what members want.