I agree with your comment about practicing and spending as much time as you can drawing. The nice thing about books, if their good books, is that they'll contain the authors/artists knowledge and tips they've learned in their many years of experience.
You may come up with a set of rules that work for you on your own, but it may take you years to accomplish this feat. Someone else may have already figured out the same thing and you can then use their method and ideas saving you time.
Other artists knowledge in whatever form, along with a lot of personal practice, is a great way to grow as an artist yourself.
Every child is an artist. The problem is how to remain an artist once we grow up.
- Pablo Picasso