Rating Range: 1200-1500
Recommended duration: 18 months
At this point you should learn more about positional play, endgames, opening principles, etc. There are several basic positional texts and everyone likes different ones.
Following those generic books, a great way to continue learning principles through game books is to start playing out ones by great players, such as Garry Kasparov's Greatest Chess Games, volume 1 and Garry Kasparov's Greatest Chess Games, volume 2 by Igor Stohl.
For the endgame you can start with Chess Endings Made Simple : How to Approach the Endgame with Confidence by Ian Snape and Chess Endgame Training by Bernd Rosen. A good follow on book is 101 Chess Endgame Tips by Steve Giddins.
At this point you can also start to learn some opening lines. FCO: Fundamental Chess Openings by Paul Van Der Sterran is the book you need. Take some time to learn a few of the main lines in your opening. One or two main lines are enough to start. Then every time you play a game, slow or fast, look up the game and find what you would do differently if your opponent played the same move. This deepens your tree slowly but very surely. Most students do not do this, but they would learn a lot more if they did!
Doing a lot of work on specific opening lines before your rating gets to 1300-1400 is likely counterproductive, except perhaps to learn to 1) avoid traps, 2) learn the first 4-5 moves of an opening, or 3) to get examples of good general opening principles.
Next page: Phase 4: Consolidation Phase