To be really honest, I don't think books are very useful, except for very specific things. Instead of wasting your money time and energy (as it's quite boring to read a chess book), my modest rules of thumb would be:
- keep on playing, that's the best way to learn... I guess one main point is still that you should stop blundering. So when playing, just force yourself to take time - especially when you are sure ; I would recommend that you try to play systematically (i.e. considering all legal possibilities for 1 or 2 moves). This is painful, but quite useful. Also, force yourself to consider all possibilities for your opponent, and not just hope it will do. This way, you'll feel that you reach a certain depth in the analysis, i.e. that you see things you didn't see before regarding a given position.
- don't do too much on openings; don't try to learn them by heart, it's too hard and useless; instead, try to remember typical moves and positions, so that you have a taste of what may work or not. (like: "I know, in the calatan, that I can play Ne5 or a4 at some point).
- learn endings: do you know by heart when and how to draw and win a pawn/a rook ending? do you know philidor and lucena positions? that's pretty useful! and you can find this on the internet.
- do not neglect strategy; I don't agree on the idea that "tactics is more important". It happens very often that people, for instance at the end of the opening,"don't know anymore what to play". These are crucial moments, as this is when blunders come.
I think people should give some specific attention to these situations where the position is "apparently equal" and there is no clear way to go.
So if you sometimes feel like this, then don't give up; tell yourself "nice! this is where challenge is", and take as much time as you need, to make sure it's the right move. You have to have good reasons to play a move!