It is used internally by pubcoder for various things, e.g. keys used to save user data (e.g. a coloring game user drawing, or the last page viewed) to localstorage et similia: if you use the same id for different books, you could end up mixing this data.
As a general rule of thumb, different books should have different IDs, if you duplicate a project, you can simply append -1,-2,-3 and so on to the id or something similar.
