URLs may have an optional path after <bookname>.
com/book-<bookname>/<sectionID A>.1/<sectionID B>.1
This tells Bookvoid to open the book <bookname> and uncollapse the first instance of sectionID A then inside this section uncollapse the first instance of sectionID B.
When browsing a book, the path in your browser's address bar is updated automatically to the latest section opened. Try it by opening and closing sections. This path is also saved automatically so that if you close and open a book, it will open to the last section you were reading before you closed the book.