What Nigel says is true, it will be easier to give an exact answer if you make the app public.
On a general note, your error suggests that you are trying to send a list of books to an element that only accepts a single book. There are two possible solutions:
You wanted to return and display a list of books
If what you wanted was to display a list of the books that contain the tag, you will probably need to replace the element you are sending the data to. I’m guessing this should probably be a Repeating Group.
If you wanted to fetch a single book, you should add the First item to your search, and adjust the search parameters to make sure the first result is the right one.