...
Pulling all of these considerations together, the simplest approach would likely be to create all resources without user-provided identifiers. The "book" would be a container and the pages, thumbnail, and METS files would all be binaries. Relationships between the resources would be established (#hasPage, #nextPage, #isThumbNailOf, etc). To facilitate more robust search, the metadata packed into the METS file could be extracted out into properties. Likewise, using a property terms that are defined in commonly used linked data vocabularies will aid in cross-web normalization.