I requested a variety of journaling and mixed media books from my library to explore. I'm fortunate because while my local library doesn't have these books, I can order them from libraries around the state. I want to recommend a few for you to look into. The first two books I recommend are:
Art Journal Freedom and Art Journal Courage by Dina Wakley. If you want to understand the principles of composition these are the books for you! Clear explanations and sample step by step projects to figure out things like the rule of 1/3rd, using white space, and repetition.
Here are some art journal pages I created based upon Dina's books. Enjoy!