Judd Books is a great place to find cheap books of shapes and sizes. Judd often takes books at the end of their print run, essentially the place buys the leftovers of print runs.
This place has a lot of great material in it, all of it very unusual. But in my opinion Judd is a particularly good when it comes to text books or biographies. Those are the books that originally start out £27.00 or around that price and are difficult to justify buying because of the cost. However if you can find the book in Judd the price will often get reduced to something like £4.00. I got a great book by Charles Dickens biography by Michael Slater (in hard back) for a very reasonable price.
Judd is also an excellent place to seek inspiration from presents. If you are buying presents for a bookworm you are guaranteed to find something they won't have even heard of let alone read at Judd and odds are it'll be a great read.