Yesterday it occured to me again, that i was asked for german literature about PostgreSQL 9.0 and the lack thereof. Okay, there is one...look for ISBN-13 978-3897216617 ;)
PostgreSQL Administration 2nd Edition is available since the very early days of 2011 and covers various topics about PostgreSQL 9.0 Administration, in addition to the first edition which covers up to 8.3 only. Seems people got confused about this.
A quick check also gives some other german book about this topic, look for ISBN-13 978-3942151061. If you have additional tips, feel free to leave a comment here.