Can it be that you are using the same set of entity classes in 2 PUs, and in one you have FetchType.LAZY on the 'library' field? _toplink_setlibrary on the Book class means that TopLink code decided to modify ("weave") your class to support LAZY relationship loading. But for whatever reason, the class that you are using doesn't match or is not completely weaved.
Regards,
-marina
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