There are some books about Gimp but as you've discovered, not nearly as many as about other programmes, and particularly Photoshop. However, as @Morpho has suggested, searching online is a very useful option here; there are many many text and video tutorials out there.

The way I am learning Gimp is by tackling projects. Start with something you want to do, and search for tutorials on that single task using Gimp as a search word. Get familiar with that task, then repeat the process for the next. And so on. Slowly your "portfolio" of skills will expand. For example, "gimp contrast tutorial" brings up an interesting looking video on using the Curves tool, a very useful tool. (Lots of other useful-looking links as well, of course, if video is not your thing or you want another approach to curves, etc.) Using the words gimp and tutorial in your search helps a lot.

In this question there are several references to tutorial lists, as well as my "starter list" for tasks that I've found useful in Gimp.

You can do a huge amount in Gimp. Don't give up :) Come back and let us know how you get on.