Recently Apple changed something to their iBookstore that has broken existing links to a publishers catalog when accessed by iPad. The author links and individual book URLs still work as before, but search links to publishers (for example, wanting to see a catalog of all the titles by a given publisher) has stopped working on the iPad. Instead you now get this message:
Books related to your search are available on the iBookstore.
You can still search with keywords for a publisher within iBooks and find their catalog that way, but that doesn't do any good if you're trying to provide a link from a website!