British Comics Sales
Interesting circulation info on the many British weeklies.

ComicBase
The longest-running and most successful inventory program for comics, Pete Bickford has incorporated circulation data into recent releases.

ComicBookPage
John Mayo has a number of tools on this site aggregating and sorting monthly sales reports. There's a lot of material here!

Comics Reporter
Tom Spurgeon's blog has many links to articles about the business side of comics publishing.

ComicsResearch
Good starting point for research with lots of links and bibliographies!

Grand Comics Database
An online project similar to the CBG/ComicBase database, this mammoth community undertaking includes justa bot every comic book you can think of — and many you never thought of!

ICv2
Milton Griepp does monthly tracking of Diamond data parallel to what's above — and publishes a lot of great info on the manga and bookstore markets.

Maggie Thompson
Maggie's beginning to post on her site many of the fanzines she was involved with; these include lots of comics history from the 1960s and earlier.

Newsarama
Matt Brady and Michael Doran's long-running news website gets into detail on a lot of behind-the-scenes information from the world of publishing. Now with a Comichron column!