Hello!
When a user visits a bookstore, the extension should check for the presence of this book in the library, and if has one, offer him or her to take this book from the library.
If the book is in the library, then we will change the icon in toolbar. Also we can show some notification to attract user attention.
Other bookstores can be added later, as needed.
If book was taken already by someone we can inform user when it will be available, of course if it supported by your API.
I will be happy to discuss the project and answer your questions (including price and term).
Thank you for attention!