This book is written to Christian husbands to improve their sex lives, though there is one chapter written specifically for wives with the stated assumption that they have already read that far too.
Given the complexity of human behavior, the description of relational problems is remarkably insightful. The premise is that for a mutually satisfying relationship a man needs to give up his rights (p104) and follow the pace of his wife, and sometimes the husband won't get what he wants (p247).
It uncovers poor assumptions and myths. For example, planning can help prepare the mood (p56, p237), and the wife does not need to be easily aroused (p304).
It gives several reasons for her lack of interest, including a husband who persistently pursues sex (p189).
It has specific advice that can be practiced. For example, when tempted to pursue, start with with emotional connection (p47) because the husband's raw eagerness pushes the wife away (p46). Then, avoid using words because words can cause the wife to feel pressured (p127). Focus on mutual pleasure instead of performance, which is a rather involuntary response.
The book even addresses common issues such as the mind wandering (p326) and headaches.