The second installment of the Composition Workshop goes straight further, where the 1st book ends, which you can get here for free: https://gumroad.com/l/KhAb
When you have mastered digital painting and know how all the bells and whistles work, the only thing that will keep you from boredom is composition. You can be as good as you want, if your work lacks a fundamental composition strategy, it is mediocre at best.
This 31 pages ebook gives a bigger insight about which techniques are important to learn and how to apply them. Along with the E-book, there are animated GIF's to show composition overlays and the explained technique better.
You can also get this book on my patreon page, along numerous other content, such as brushes and image reviews, feel free to join and support my ongoing work here: http://www.patreon.com/fantasio