Here's the exact link for ePub package metadata.
http://www.idpf.org/2007/opf/OPF_2.0...tml#Section2.2
It's part of the package specification and most reader devices show the metadata, at least the big Five. Title, Author (Creator), Identifier, Publisher, Date. Standard DC (
Dublin Core Metadata - the big international standard for this stuff) is allowed, and all metadata fields can repeat. In addition DC supports Contributor, Description, Rights, Subject, Source, Extent and a number of other arcane fields and attributes. All these can be included. How much of this is available through a reader interface is the decision of the reader developers.
There is one big over-riding rule. There has to be one Metadata identifier that is referenced from package unique-identifier attributed. There also must be at least one Title.
Ultimately metadata can be as big as you want, use various DTD schemes, for example Metadata on age suitability, learning descriptors, or just put your own MD fields in using the HTML method <meta name="Cover-Colour" content="blue" />
but it gets pretty techie to implement. Anyway, there's plenty of Metadata to go around in ePub