There's a great exposition of FRBR (the Functional Requirements for Bibliographic Records model "work -> expression -> manifestation -> item") in this post from The FRBR Blog on De Revolutionibus as described in The Book Nobody Read: Chasing the Revolutions of Nicolaus Copernicus by Owen Gingerich. See post for the background and here (103 KB PNG) for a map of the FRBR relationships.

(Yes, and a twinkly star in the title too. ;~)