Well, it depends what knowledge you have of the market, what instruments you wanna trade and how you are gonna trade your market of choice.
I, talking for myself, never bought any book but had already basic economic background and collected as many info I could from the web and business tv. Generally info available on books are available on the net and they cost you nothing.
I never felt the need to buy books, but on my side I had as much time to dedicate to learning as I wanted.
This doesn't mean that books are good for nothing but there's a lot of trash out there and you need to be careful no to waste $$.