There are many aspects to becoming a strong player.
One of the very best books to read is called "Think like a grandmaster" . You can find it at Amazon. It helps you how to think. Chess is about analysis, and this analysis is done in the mind. Hence if you know how to organize this thought process, you'll become more efficient and actually can analyze more positions in the same amount of time.
Another thing to do, is to get good book with good analysis. Like Fischer's my best 60 chess games, or Alekhine my best games, I mean a book that is annotated by the same strong player, and this way, you can see the thought they had when making moves.
And most importantly, play with players who are stronger than you. Losing teaches you a lot. Make sure you go over each game you played afterwords and try to learn why you lost, or even if you won, see if you might have missed better moves.
Getting good at chess is not easy (I mean to go over 2400 level or so), it is just hard work. Need to study a lot, read a lot, and practice a lot, and have some natural talent for the game. But make sure you are having fun doing it, else it is not worth it.
Just noticed that an important information is missing in this question. What is the level of the player here?
Advice to someone at 1500 wanting to improve to 1800 will be different from someone 1800 wanting to improve to 2200 level, and that will be different from someone at 2200 wanting to improve to 2400 and so on. For example, someone at 2200 level who wants to get to 2400 level, they have to spend more time on studying openings, more end game positions, play more games in stronger level international tournaments but may be spend less time on tactics since this is something they should have good handle on at this level. While someone still at 1500 level, the situation is different.
So hard to give one generic advice without knowing what level the player is at now.