Editing and layout aside (and it is truly awful, as detailed in the above link), the contents are (estimated page counts, since there isn't a TOC):
About 20 pages on the creation of the game and the "early" days of D&D/AD&D. About half of this space is devoted to the history of the GenCon tournament. This is the only section of the book with any art from prior to 1984. There is a 1999 essay from Gary Gygax, which I believe originally appeared in a 25th anniversary product.
About 100 pages devoted to the various campaign settings. FR, DL, and PS seemed to get the most ink, but I didn't count. GH only got 6 pages. The vast majority of the art in this section is from the mid-90's or later. The Mystara section, for example, only had two small images from its pre 2e-days, with all the rest of the art being from the half-dozen or so 2e products produced in '93/'94.
The next approx. 50 pages are devoted to the 2e years. The first half of this is devoted to the creation of 2e, with the second half focusing on some of the products of the era - the green historical supplements, the blue DM helpers, the red "Complete" books, and the '91 Black Basic Box.
The last 85 or so pages deal with WotC's acquisition of TSR and the creation of 3e.
Intersperced throughout are essays from semi-notables on how D&D positively effected their lives.
Personally, while I didn't expect a critical history of the game, I did expect a nostalgia piece like this to show a little nostalgia for the early history of the game. Rather, it got the first half of the game's history out of the way in a prefunctory fashion in the first 20 pages. There are also some very odd decisions with regard to what products are focussed on. There is no section devoted to adventure modules for example, but over a third of the book is devoted to settings. Another example is the 5 or 6 pages given over to the 1991 Black Basic Box, which is very odd given that it was fairly unpopular and that the D&D Rules Cyclopedia, which came out a couple months later, has proven to be the product in that line that fans still talk about. Overall, it paints a very odd picture of the history of the game, and one that is very skewed toward very recent events. In my opinion, the only reason why someone might want to get this is if they want a coffee table book full of the art of D&D from the last 10 years.
R.A.