In the authors questions he said he would create a site with background information on bartimaeus.
Can someone post the URL for me? :(

The ask the author section is for asking questions directly to JS. Moved.

http://jonathanstroud.com/
http://www.bartimaeustrilogy.com/

Are really the only two official sites. There some narrative by Barti on the first one, but nothing written about specifically about him, I don't think. Its worth exploring anyway.

The first one actually had a lot on Barti when I visited.