The biggest difference between intermediate play and beginner play is that not all games are (or could be) won on tactics. Sure, if you are a better tactical player you are likely going to win no matter what your rating, but there are many games played at the 1600+ level where the winner actually wins the game without the losing player just making a bad tactical mistake. Note I did not say “most” or “all”!
Tactics still predominate, so don’t forget to keep studying them. So first let me recommend an excellent tactical text: The Art of Attack by Vukovic. This is a classic work about how to probe for weak points, the idea behind the classical bishop sacrifice, etc. Highly recommended.
If you started as I suggested and have done all of this then, with a firm foundation on the basic motifs, you should be doing tactics at around the 2000 level. All of this board vision cannot be absorbed and assimilated in 6 months but, in conjunction with playing lots of slow games, you can get there in 2-5 years.
Another overlooked point is that while many books are on offense, just as often you are on the other side playing defense, so How to Defend in Chess by Colin Crouch is a worthwhile text.
A study is typically an endgame puzzle that has a surprising combination of tactics to win. Studies are usually created by a study composer. Solving studies are excellent tool for improving your analysis, calculation, visualisation, imagination and endgame skills.
We highly recommend solving studies on a regular basis. The two greatest composers for endgame studies are Troitzky and Kasparian. A good place to start is Domination in 2,545 Endgame Studies by Ghenrihk Kasparyan. And another excellent resource is Collection of Chess Studies by Troitzky (in descriptive notation).
We also recommend The Art of the Endgame - My Journeys into the magical world of endgame studies by Jan Timman. It has knight forks, tricks, mutual zugwang, under-promotions, combinations, stalemate patterns, rook vs dishop and more.
Another three excellent endgame books are:
If you need an advanced general endgame manual, Understanding Chess Endgames by John Nunn is a comprehensive read.
Next page: Phase 6: Becoming an Expert