I received a review copy of this novel from Sourcebooks. This in no way affected my opinions.
Georgette Heyer's Regency World contains a wealth of information about the Regency era. It can be used for more than just Heyer's Regencies, but any Regencies.
I read it once through, but this is the type of book that can be used for reference. It's separated into countless different sections, so it's easy to look up something real quick that you come across in whatever Regency you are reading. Sections having to do with fashion, leisure, sports, travel, food shopping, etc. Not to mention a brief bio of all the major players during the era and a glossary of common phrases used. Reading a Heyer and a character is using slang that you have no idea what the devil he/she is talking about it? Look it up here!
This is a handy dandy book to come to again and again for a quick Regency refresher course.