There's a host of books, get a general opening book - one that explains what you are trying to do in the openings not one that just gives loads of variations. You will find that just understanding the general principles gets you a long way in this phase. For the middle game there is a book called something like Master vs Expert (and one called Grandmaster vs Master); a friend that I have last touch with had it and I found it useful, unfortunately I can't give you an ISBN number or tell you who wrote it. A book on the end game is a must. A lot of games are won and lost in this phase.
If you only want to buy one book get something like Bobby Fisher's my 60 Memorable games - most of the top players of history have one, so pick the world champion you would most like to play like and buy his book.
Basically what I'm getting at is at least to start with go for something general that explains the ideas.
Something that I've noticed about a few games I've played against less experienced players: Avoid putting knights on a3(6) and h3(6) in the opening even if you want your bishop on the long diagonal - it is almost always the wrong thing to do, they are no use there; they want to be watching over the central squares from f3(6) or e2(7) and d2(7) or c3(6). When on f3(6) they also watch over the h pawn and stand in front of the f pawn both of which are likely targets for your opponent.