In September/October 2006, Penguin Books published the 8th Edition of The Penguin Guide to Jazz Recordings by Richard Cook and Brian Morton. Like its predecessors -- I have a shelf with all of them -- it is the most invaluable of all jazz guides.
Each edition produces a set of differences. The additions come out on a two-year schedule, so each new one adds new records. To compensate and conserve space, Cook and Morton try to weed out records that are no longer in print, and on occasion that results in whole artist entries being dropped. I started plotting these changes out beginning with the 5th edition.