Good Websites for Buying Books

I'm planning to make my OH a "book box" for Valentines day. I gave him a metal bookmark for Christmas that has 50 must-reads printed on it and he's determined to work his way through the list. In his work, he gets frequent five minute breaks where he has to sit and wait for somebody else before he can get on with things so he likes to whip out his book and read it for five mins - as a result he's always reading and gets through books pretty quickly. But he isn't very well-read so I thought I'd put together a selection of books for him.

I've already bought some classics that are only £2 but I need more books to complete the gift. Due to the fact I want to buy a lot of books, I'm restricted in price and can't afford to buy too many expensive books. But I don't know any decent websites. I've trawled my way through Amazon and found some bargains but I need more. Does anybody know any decent websites for selling books or anywhere I might get some bargains? There is still a branch of The Works open near me so I'm planning on popping in there plus I'll have a look in the supermarkets. I don't normally buy books online and, when I do, it's from Amazon or Play so I don't know where else to try ? ?

Thanks in advance