You can occasionally find Unofficial Guides from the late 80's on eBay - those are a lot of fun to read, especially if they pre-date the Studios opening.
FWIW, the Guide is updated twice yearly: an August edition is the first printing of the following year's guide, and an update is published in February of the following year. Thus, the 2011 Guide will first be available in August, 2010, with an update in February 2011.
Somewhere between 10 and 20% of the Guide changes every year, including hotel, attraction and restaurant ratings, plus reader comments. The chapter titles and book layout are fairly stable at this time, which is why (I think) people think it doesn't change much. The actual content, however, does change.
For the 2011 edition, for example, I submitted close to 100 pages of updates out of 800 pages of text. And I'm one of several people who work on the book.
Thanks for reading the book!
Len