The idea of dividing a longer book into sections is a good one, especially if it is for upper elementary school readers. They get a feeling of completion not only from each chapter but also from each section.
So how do you make those sections stand out? Well, for me, I tried doing it by giving each section a special page with a special title. The idea is to make each section feel like a shorter book with each section having its own key full-page illustration.
This is similar to the way musical animated cartoons are planned. The songs occur at important points in the story and help to tell the story all by themselves. Here, the key illustrations do the same with a little help from the title.