Good Book on PostgreSQL?

[Matthew Tedder]

Can someone recommend to me a good book on PostgreSQL?
Although I am mostly a DB2 fan, PostgreSQL will enable me to offer my healthcare apps 
to low-cost centers such as Hospices and low-income community medical facilities of 
various sorts.  The value in this is getting a new app tested and matured enough to 
sell it to larger commercial clinics and hospitals.

--Matthew