Trying to use a pack of sites and they usually fails.
It happens for any book, but for example lets take the book BATTLESHIP BISMARK
1. Barnes and Noble plugin results in error 403
2. Loc.gov results in error 503
3. Libex.ru results in error 500. the second search works, resulting in nothing and it's normal. But for any book the first search gives error.
This problems exists for a long time and doesn't change.Added after 26 minutes 23 seconds:
One more strange thing found
Adding: Polikarpov I-15: Le Guerrier au Nez Camus book
Amazon.com plugin doesn't find it!