This is a web-based resource rather than a book, but the majority of it is written articles, so I thought it would be relevant here.
The organisation "The School of Life" is run by Swiss-born British philosopher Alain de Botton. There are many videos and articles which you can view for free on e.g. YouTube; I subscribe to the app, which has a large amount of content. The material is divided into categories such as "How To Know Yourself", "How To Heal From Trauma (parts 1, 2 and 3)", "The Secrets of Successful Relationships", and many, many others. There are articles on sex, relationships and affairs, as well as quite a lot of background about psychotherapy.
I'm using these materials as part of my affair journey (I'm a BH), to help me understand more about myself and my WW. The articles are quite gently written and are non-judgemental; they're probably not designed to help you confront the more unpleasant details of affairs, but they do help you to create a landscape of compassionate context - either for yourself or your partner.
Many of the articles relate to the way in which our childhood forms our character; for example, de Botton talks about attachment styles in some detail, and discusses the ways in which our more irrational behaviour can be linked to wounds we received when very young. In lots of the articles, he will give hypothetical, yet very specific, examples of the kinds of problems we face - the horror we feel at the way our partner eats, or how they embarrass us when they tell boring anecdotes at parties - and helps us to try to understand why these things bother us so much.
He talks about the importance of expressing oneself - through crying, screaming, dancing, listening to music - and tries to explain how and why some of us become very inhibited about such self-expression. He advocates for compassion for all people, whilst never losing sight of the fact that another's actions can hurt us deeply.
The articles vary in length: the shortest take about a minute to ready, whilst the longest are around half an hour - but the majority are about five minutes long. The videos are delightful, as they are accompanied by beautiful, quirky and imaginative animations; most of them are narrated by de Botton himself, and his voice is very easy to listen to.
A subscription gives you access to a huge amount of material - I got a special offer for £29.99 (GBP) for a year and I think the normal price is nearer £50, but I will seriously consider renewing when my subscription expires, as I think there's enough high-quality material to justify the price.
The School of Life will not solve your problems and, if you're in a similar situation to me, you'll still need therapy and difficult (very difficult!) conversations with your loved one, but I find that, reading a little each day is bolstering my confidence and giving me strength to take each step.