A book consists of chapters and sections arranged in a treelike way. You can see this treelike table of contents in the left part of the main window. The window for viewing a chapter or a section is on the right. To read some chapter, just click its name in the tree on the left. If a chapter has sections (subsections), click + to see the list of them. You will see the list of sections in the selected chapter. Click the name of a section to see its contents.
You can use the mouse or the keyboard to select a chapter or a section. Use the up arrow and down arrow buttons on the keyboard to view the previous and the next chapters respectively. The right arrow button will open the list of sections (if the selected chapter has any) and the left button will close it if it is opened.