Things I didn’t know: you can do an empty search on Amazon, narrow it by price, and then choose a category in the side bar. Which means you can find, for example, all the poetry books currently available for £0.01 (such a seductive price, even though it really means £2.81, a price which would make me think twice about buying a book in Oxfam but has me merrily clicking Add To Basket, or indeed Buy With One Click, on Amazon). If you look at everything available for £0.00 you can see the amazingly obscure things available for Kindle.
Incidentally I’ve often thought that someone should map the speed at which individual books fall to £0.01 on Amazon. It must be related to two factors: inversely to the quality of the book and directly to its immediate success on publishing. Even really good bestsellers fall to £0.01 very fast; but terrible books which sell moderately seem to fall to £0.01 much faster than decent ones with comparable success. Someone should test this theory! It would produce a lot of attractive downward pointing graphs, too, which is always nice.
And of course there are some books which just get more expensive all the time.