Currently, Magic Books will return an error when attempting to set a new cover using any means, whether title, ISBN, or other identifier.
This is a Linden Lab issue. I have filed a bug report and will be following up on it with the Lindens. The same issue is also impacting a number of other items (including the Trivia Globe and Kyrie) as well as a number of private services I have built for others.
Until Linden Lab fixes or explains/documents the unspecified changes they have made to objects requesting data from external web services, the Magic Books have been withdrawn from sale in-world and from Marketplace. Thanks a lot, LL.