Today, I may take my notebook and go to a good library branch to survey their gardening books. If they are older than three years, it is highly likely I can find them for sale on the web as used books. I have found one place where no postage is charged and they have many books starting at $3.48- that's the total investment! I will go to the library at the botanical gardens today and do the same thing. Of course, I don't NEED more books like that; but it is an economical indulgence.
I have noticed that my nearby used book store seems to over price both cookbooks and garden books.
The postage free book site is http://www.betterworld.com; they actually manage to donate money to literacy councils all over the country from their sales. It's a win/win situation all the way round.