Clearly the cheapest source of books is your local library. If you ask nicely, usually they'll even order a book for you. They'll certainly find out whether the book you want is in another local library and transfer it for you. Our local library even gives away the books it no longer wants to stock.

Next cheapest source is charity second hand book stores. We have one next to the local station which sells practically brand new paperbacks that would have cost £10 for 50 p.

Another good source of second hand books is Abebooks:

http://www.abebooks.co.uk/

which covers 13,500 booksellers selling 110 million books.

Though, sad to say, AbeBooks has now been bought by Amazon, so you might want to have a look at the following link before purchasing:

http://business.timesonline.co.uk/tol/business/industry_sectors/retailing/article5337770.ece

Markets often have good value bookstalls. The Old Market at Spitalfields for example has an excellent discount bookstall selling a more interesting range of books than you would typically find in a discount bookshop.

If you're going to buy a new book online, make sure you're getting the best price by using BookBrain:

http://www.bookbrain.co.uk/

I have noticed recently when using bookbrain to look for books that the Book Depository is almost always cheaper than Amazon, and they offer free delivery on all books, even those under £10.

http://www.bookdepository.co.uk/

The Book People offer a selection of new books at very substantially discounted prices:

http://www.thebookpeople.co.uk/

Scholastic also offer big reductions on children's books:

http://www.scholastic.co.uk/

Lulu.com is a company which enables authors to publish books in small print runs without having to rely on the big publishing houses. They sell both paper books and e-books. The physical books aren't usually that cheap, but quite a lot (though by no means all) of the e-books are free:

http://www.lulu.com