There is no one specific book for me. When I first started Phil Gordon's "little green book" help a lot with some poker strategy
but like HOH it is mainly for MTT's. Collin Moshman's "Sit n Go strategy" was very helpful when mastering stt's but that strategy isnt as effective as it use to be. Jared Tendler's "the mental game of poker" was also great because it taught me that even great players make bad plays sometimes.
Anyway to summarize there wasn't any one book that unlocked the key to poker but each played an important role in my development.