Currently the extension doesn't seems to respect the max number of articles which a book could have ($wgCollectionMaxArticles = 500 ).
You can confirm this by going to a big category of Wikipedia, with more than 500 articles (e.g. [[Category:Living_people]]), and adding it to a collection. The extension will add only 500 pages, but then it should display the warning from [[MediaWiki:coll-limit_exceeded_text]] ("Your book is too big. No more pages can be added."), and it doesn't.
Besides this, if you go to another article wich is not in you big book (e.g. [[Sport]])), and add it, your book will have 501 pages and no warning will be displayed.
A user can also easily create a book page ([[Category:Wikipedia books (community books)]]) with more than 500 pages and then load it into [[Special:Book]].
This inconsistent use of $wgCollectionMaxArticles should be fixed.
T71003: Book with 3000 entries failed rendering