RawPedia is the only official source of documentation for users.
Read RawPedia here: rawpedia.rawtherapee.com
Developing raw images is a breeze when you know what it is you want to achieve, and when you know the tools you have at your disposal. The RawPedia wiki contains articles which describe every tool and topic related to RawTherapee, and more. Reading it will answer most of your questions, it will give you greater power and maneuverability in getting the most out of RawTherapee’s tools, and it will save you time!