Lately, I've been scrapping out the used books section for amazon and it's very easy to find bargain for older books. The last two books I've bought are:
1-: Medical uses of statistics, second edition, by John C. Bailar III and Frederick Mosteller which was bought for 0.01$ + 6.49$ for the shipping and in very good condition.
2-: Interpreting the Medical Literature, 4th edition by Stephen Gehlback for the price of 0.52$ + 6.49$ for the shipping and it's described as like new.
I think I'll start collecting such bargain and improve my bookshelve.
Quick advices, when looking for good books to read in a particular section or after a search, sort out the search according to the price (lowest --> highest) and you may find a few gems in the list for a few bucks at most.