Have a look on eBay for some bargain books - someone selling a job lot that you can pick up locally or a couple of 50p or a pound ones with cheap p&p. If it's not for you you can either sell them on or drop them off at the local charity shop.
I've just bought 20 psychiatry books off someone that's having a clearout -each one at the starting price of 50p! Got to pick them up from Trafford Park so will tie it in with a trip to the united museum with the kids during half term. Sorry, I mean War Museum ;-)