There have been several articles about JDO on various websites. Some are listed below
SUN : Oracle JDO Web Site
TheServerSide.com : discussion on JDO and Entity Beans
TheServerSide.com : JDO 2 Queries - Methods, Paging, Unbound Variables
TheServerSide.com : JDO 2 Queries - Projections, Aggregates
DevX.com : JDO brings DB Programming into 21st Century Despite Controversy by Dirk Bartels
Java Data Objects by David Ezzio
A number of books covering JDO are available. Some are listed below
http://www.oreilly.com/catalog/jvadtaobj/Java Data Objects] by Craig Russell and David Jordan. Publisher: O’Reilly
Core Java Data Objects by Sameer Tyagi, Michael Vorburger, Keiron McCammon, Heiko Bobzin. Publisher: Prentice Hall