ZODB HOWTO, by Michel Pelletier:
Goes into slightly more detail about the rules for writing applications using the ZODB.
http://www.zope.org/Members/michel/HowTos/ZODB-How-To
Introduction to the Zope Object Database, by Jim Fulton:
Goes into much greater detail, explaining advanced uses of the ZODB and
how it's actually implemented. A definitive reference, and highly recommended.
http://www.python.org/workshops/2000-01/proceedings/papers/fulton/zodb3.html
Download link for ZEO:
http://www.zope.org/Products/ZEO/