For psychology, as Wizardtop suggests the Gross one, and as an inexpensive one Psychology A2, The Complete Companion by Mike Cardwell and Clara Flanagan, First edition (don't bother with the more expensive 2nd edition, the 1st one is much better). I used this book more than any other book, and for the £2.50 it cost me from Play.com it is a must, it doesn't have depth, but it is great as an overview, I found I used that for majority of my essays and then used the Gross book for more depth. Another overview book that was great for me to read and understand things was Advanced Psychology through diagrams by Grahame Hill.
Personally I would advise Play.com over Amazon, for second hand books, mainly because you pay a little more for the book but postage is free, but with Amazon second hand books from other suppliers you have to pay postage on each item so works out dearer if buying several books from different suppliers.