I found some books about RawTherapee on Amazon, but I think none is in English. Probably the most comprehnsive one is in French. Then I found 2 German books that are partly on RawTherapee - some chapters in a book on GIMP and one that is about developing RAW files in general. And they are all not so new.
I guess, books are also a question of money - e.g. there is already a lot of literature about Affinity, although the program is younger than RawTherapee. Altough, I find it hard to believe that a powerful program like RawTherapee can be developed just by some “amateur” programmers in their free time, who are not getting a cent for their work. I imagine that there are hardware (camera?) producers who are supporting RawTherapee.
Anyway, it is a pity that there are not many books about this program, from my point of view.
Then, there are also quite a few books about GIMP…
I know, there are tutorials around in the www… still, it is not the same as a book (that you are willig to pay for).
Is there any work in progress here?
Regards from Vienna