The earlier versions of the plugin worked by using search.overdrive.com to detect books, which were then checked for availability at individual libraries. This could detect books that are known to Overdrive, but could not determine whether they were available for purchase by a particular library.
There were some problems using search.overdrive.com in the plugin. It is fairly slow and newly available books do not show up there immediately. I also came across several books that the search would consistently fail to locate, even though I knew they were available on Overdrive.
Since version 1.7.0 the plugin no longer uses search.overdrive.com, but instead performs all searches at individual libraries. It uses the advanced search option "Additional titles to recommend" when available at individual Overdrive library sites to find books that can be recommended for acquisition. I don't know how Overdrive determines which books appear for this search type, but I know that the results are different for different libraries.
Overdrive has a "feature" that if you use search.overdrive.com to locate a book, such as High Hunt, you can find the Overdrive content reserve ID for the book and use that pull up a page for that book at any Overdrive library. For example:
http://freelibrary.lib.overdrive.com...c-0d244012cef4
If the library doesn't have the book a "Recommend" button will be available, but that doesn't mean that the library can actually purchase the book.