Which book to buy?

[Nightlight87]

I've read a few books so far including Getting Started in Holdem, Small stakes holdem, Harrington on Holdem I and II, and the Little Green Book.

I play microstakes NLHE, recently moved from FR to 6max if that matters with the very rare SNG thrown into the mix for variation.

I recently came into a bit of extra cash and decided it was time I attempted to improve my game a bit.

Has anyone got any recommendations for what book(s) I should buy? I was looking into Professional No Limit Holdem, would this be a good choice?

Thanks