[Jeff76]

For NL (since that what you asked), All three HoH books. I don't think either I or II is better than the other; they are both necessary for the game. And III fills in a lot of the post flop play that I felt was missing in the first two volumes.

SSHE should be mentioned, though, even if it is limit. Just an incredible book that puts money in your pokect. Reading it did help my NL game, too, if indirectly.

And of course, ToP just because those fundamental concepts are so key to playing winning poker.