I added the books with the ISBN 349701754X and 3608940677 to the BookLibrary inside my Joomla website. Both book are now visible in the BookLibrary::BookManager with all proper information retrieved from amazon.de
Next I installed the Book Library Link-In-Content 2.1 plugin and placed the following code inside an article:
For the Book with the ISBN 3608940677 I get "Book not found!".
I discoverd that all books in the library with an ISBN ending with the character 'X' can be shown in an article with the booklibrary isbn=..... code and all the other books not.
I am using BookLibrary 2.1, Book Library Link-in-Content plugin 2.1 and Joomla 1.7 (german language) and Amazon.de as source for the books.