The Browse-by-Query Language manual now has cross references between each object type and the operators relevant to that object, making it far more usable: http://browsebyquery.sourceforge.net/bbq_lang/html/index.html
I've also cleaned up some errors in the documentation.
The language manual is also now available as one big HTML page, which might make searching more convenient: http://browsebyquery.sourceforge.net/bbq_lang/html/bbq_lang_1_page.html
If you want to know more about the ideas behind BBQ, you can check out:
All these are available in the SourceForge documentation list for BBQ.