I often get emails from students asking where to start when they want to improve their paintings. I will try to list some recommended books from time to time here.
Composition is such an important element to making great paintings. So my first recommendation will be Composition of Outdoor Painting, by Edgar Payne.
This book covers several types of compositions and gives great visual examples from several artists, besides Payne. Though this book is geared towards outdoor painting, the composition principals apply to all types of subjects, from still life to figures.
A strong composition will help a good painting become a great painting.