from a personal point of view, i find books of limited use - far more valuable to me are posts such as the ones found on here.
speaking as one whose maths head is firmly wedged where the sun don't shine, i find even getting far enough into these books somewhat of a struggle and of course, generally they are coming from only one, perhaps two, points of view.
admittedly, these will be from extremely successful players, but why bite off more than you can chew? unless you're a born poker AND maths genius, a variety of input from real, more tangible people, such as those found on THF, with whom one can at least interact, ask questions of etc, is of potentially far more use than trying to put a load of written theory into practice. people on the whole assimilate knowledge much more effectively from their peers and from personal experience than they do from some disembodied and far removed expert. which isn't to say that that outside knowledge isn't useful - but in that case, i think actually observing these people play and THEN analysing it has to be more effective than just reading about it. the visual aspect of seeing these theories put into practice is so important for remembering them in the first place, i reckon!
also lacking from the books is the aspect of debate! you only ever get that one point of view in a book, whereas here, for instance, you get so many others' 2c worth that you can form a much broader picture of whatever is being discussed, make up your own mind and incorporate what you've learned into your play, then rinse and repeat until you find what works. like CardLovinCat suggested, it can be too easy to get locked into one way of thinking. surely a successful player (who knows, maybe one day i'll actually BE one and be able to deem myself qualified to stick my oar in!!
) is one who can be flexible, mutable and one who is not so embroiled in the theory of what they should be doing as to forget to actually do it!!! it's no good sitting at a table, thinking, 'ah, well, so and so says i should play such and such a hand THIS way from THIS position' when the table you're sitting at and the players you're up against are in all likelihood very different... you may be able to read a book, but you can't READ a book - know what i'm sayin?!
yet another aspect where the books are less useful i think is that the guys writing them are usually talking about playing against opponents who are thinking along the same lines as they are. this is great if you have a fairly level playing field, but playing odds/position etc is definitely trickier when you're up against someone who frankly doesn't have a clue!! it's all very well making a text book play with the right hand from the right position but if the maniac sitting next to you just shoves with anything at any time, book theory becomes somewhat moot. how many times have i folded what would have been the winning hand, or gone, 'f**k you' and CALLED with the better hand only to be outdrawn?
probably i would gain a lot more from the books were i playing at a higher level, but at the levels i can currently afford to play at, i'm learning far more from you guys/my opponents than i am from the little heap of partly-read, vaguely understood and promptly put down books currently making my bookcase look pretty!
for me, they're great for learning about the existence of other aspects of the game... which i then look for IN ENGLISH and learn about elsewhere/in practice! playing by the book is all very well, but in my humble opinion, nothing takes the place of instinct/feel/read when you're actually at the tables. so, i may not be able to define fold equity or other such terms to anyone, but it'll come - not because someone has explained over and over ad nauseum what they are, but because i'll have seen and experienced the situations in which they occur to the point at which they finally sink in.
my bookcase does look VERY pretty, it has to be said 
