Haha, what a crazy funny bug!!! Sorry for my false accusations.
But I'm still not clear how the sign up page is included. Aren't you searching a database based on keywords and generating pages on the fly? What do you mean by "that page will be included by the search function"? I don't see any dedicated pages for individual books.
I use a customized Wordpress instance as the front-end for the website. Worpress search includes both posts and pages by default. I fixed the bug by excluding pages from the search with a filter function. Now only archived newsletters will show up when you search for a given keyword.
In the future I plan to make the search a much more prominent feature, sort of like a search engine for new books. At that time, I will use a backend which will contain the individual books, rather than simply do a text search through the newsletter archives.
But I'm still not clear how the sign up page is included. Aren't you searching a database based on keywords and generating pages on the fly? What do you mean by "that page will be included by the search function"? I don't see any dedicated pages for individual books.