I second Wild Rivers advice, "tournament poker for advanced players" is an amazing book, which helped & continues to help my MTT game no-end. Ive read it front-to-back a few times, and reread a couple of chapters and I always find something I hadn't fully taken-in the previous time.
For MTT's it is the bible and slansky is the god!
For cash strategy I would personally suggest Doyle Brunsons Super System (the original). It was written 25 years ago but still beats anything Ive read written since. You just have to ignore some of the out-dated info.
hope this helps
PS: I got hold of an ebook copy of one of phil helmouths books and was very dissapointed, not a good read but I cant remember what it was called.
PPS: Julyangels, most of the references still apply and yes I would say if there are holes in your game it can be a very wise investment. There are lots of books based around online play, such as "make millions playing online poker" by matt goldman (definitely goldman, think his first name is matt). which is a great book full of solid advice and all the maths any player will ever need.