Suggested changes to Book pages

http://www.postgresql.org/docs/books/

Suggested changes based upon feedback from publisher

* Each book should have "Language: XXXX" as part of its metadata, so
people don't waste time looking at books not relevant to them

* The books should be arranged by version, so we have Version 7, Version
8 and Version 9. 

* "PostgreSQL Reference Manual - Volume 1-3" is actually the manual for
PostgreSQL 8.2, so we should mark that clearly to avoid confusion.

* The Douglas' book is mentioned twice, for both versions. Nobody wants
to buy the first version anymore, so we should remove that.

Thanks,

-- Simon Riggs           http://www.2ndQuadrant.com/books/PostgreSQL Development, 24x7 Support, Training and Services