There aren't many books for the oil pastel artist out there. This one and Oil Pastel for the Serious Beginner: Basic Lessons in Becoming a Good Painter are the two that I found recommended again and again when I started using this medium. Both have great information on history, materials, and techniques. The big difference is that this book has examples of many artists' work in it, in a variety of styles, while the Serious Beginner book has only the author's work shown. With only two major books on the medium, you should definitely check them both out.
There indeed very few resources in this medium. This specific book I've picked up as I failed to anything remotely decent about oil pastels on YouTube. Who could have thought, eh?
Having had quite some experience with the medium, quite a lot of things were already obvious for me. But I still found the book a good read, as there were 1) helpful tips tricks; 2) amazing range of other artists works; 3) a personal touch in the writing. Given how short it is, I would recommend it even to people with some experience.