Our couples therapist recommended this book and it's been very helpful:
My only complaint would be that the little case history stories never seem as bad as what we're going through. But it's very balanced and lets us both feel heard and understood. We've actually been reading it out loud to each other and working on all the exercises together. It's very good.
Probably best, though, for the early stages of having learned of an affair. If anyone has questions, I'd be happy to answer them if I can.