Book Purchase Help

I've been playing NLHE for about a little less than a year now. I'm not that horrible but I really want to improve my game, and I don't think any basic book would help. Usually I play NLHE single table tourneys with friends, cash and multi-tables are occasional too. If I could get 1 and only 1 book (I'll eventually get many but just to start...), which would be the best choice? Books like Harrington on HE, SSHE, and TOP seem to be popular, but I want everyone's opinion. Also keep in mind I pretty much only play NL, and I thought SSHE was for limit.