By default, only the first 16 pages are rendered. You can make the size bigger using the #maxpages command.
You can also see a scrollbar if the HTML page is larger than the page defined in the PDF. Tables maybe have a fixed width that exceeds the page size. Adjust your page setup settings to make enough place for the elements.
HTMLBook is a GUI application, but you can use the Open command in the terminal. If the file has a #fileout command, the file will be previewed and exported automatically and the programm will quit. The file must have the filetype ZSIH or the extension .htb.
open myhtmlbookfile.htb
Note: The application must not be open when you open the file. The user must be a user with GUI-access.