Can you give me some examples where it failed? I need to look at them to work out why it failed to find the book.
One common reason for not finding the right book is if the title is too common. The plugin is only using the title for the search. And only looks at the first page of the results. For a common title, there can be a lot of books found, and, the one you want is always the last one.
I will have another look at this. When I was developing it, adding the author didn't help the results. But, I just did a test on the site, and it did help. Maybe they have changed the search.