I seriously doubt that the differences between revisions of Jackson's book would be worth worrying about (four years difference). Don't forget about half.com, you can save a lot of money on books. I'm seeing $0.75 + shipping on the 1993 book.
Half of the exam material highlights the technical aspects of brewing, troubleshooting, and preventing process problems. The other half focuses on style guidelines and tasting/judging beer. The technical, trouble-shooting, and process questions are more easy to prepare for; every possible question is given in advance in the study guide, so the exam is about as "open book" as a "closed book" exam can be.
The Exam Study Guide documents are available here. These study guide docs are packed with useful information for brewers of all levels, so even if you're not studying for the BJCP, it's worth a look.