If you had to get one...

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I am going to get another book... I don't have much start up money at all, but I love to read. But I want to improve my playing as much as possible so I'm getting one book... I just want some input on which to buy first if that's all I can get is one:

Gambling Theory and Other Topics
Getting the Best of it
Small Stakes HE
Harrington On HE

I want something in depth and I don't mind hard reading. Which one of these should I start with to get me on the road to having enough money for all of them?? (Eventually I will get them all... I just can get one right now)

If this helps: I normally play .5/1 or 1/2 NLHE