I am currently reading The Art and Science of Valuing in Psychotherapy by Dahl, et al. Thus far (about halfway through it), it has been very accessible and engaging. It provides a nice synopsis of RFT as well as the basic tenets of ACT and does well to clarify different types of cognitive fusion, which would seem to be a helpful way of conceptualizing fusion with clients (or with ourselves). I would recommend this book to anyone interested in ACT, though like with most ACT books, it is highly advisable to read the original ACT book (Hayes, Strosahl, & Wilson, 1999) before you do so.
On a similar note, I very much appreciate how each ACT book that I have read thus far provides a summary of ACT & RFT in general. Though I am familiar with these concepts by now, repetition helps to solidify these concepts even more. As ACT is a somewhat unconventional approach to psychotherapy, this repetition would seem to help provide more exposure of the basic concepts, but also to expound upon them by exploring the various domains of the ACT Hexaflex. For example, I am reading the Values book right now, and Mindfulness for Two (Wilson, 2009) is next in the queue. I am also wondering if someone out there is working on a book which tackles Cognitive Fusion in similar fashion.