I would recommend any book by CJS Purdy. Probably the most enjoyable is The Search for Chess Perfection. Purdy had an uncanny ability to explain the game without using a lot of variations; even Booby Fischer praised his analytic ability. It might be of interest that I gave a copy of the sections of the book on how to reduce errors and how to plan to a mid-1600 player and in about a month he was playing at the 1800 level.