It's the same size as their Star Wars character encyclopedia (19x24cm) and follows the same format. Its 140 pages cover 'every LEGO Harry Potter minfigure and creature' so the scope is more than just minifigs although they of course form the bulk of the book. It's divided into 7 chapters, one for each movie. This makes it interesting to flick through, but if you want to compare every version of Hermione, for example, you have to hunt back and forth through the book to find them all.
Pages containing minifigs include facts about the figure in 'pull-outs' from the main photo, and a 'data file' that lists the sets that the figure came in along with any accessories included with it. If there are variants of the figure these are shown in a 'variants' box, so, for example, although there have been two Dobby's there's only one page for him: the old tan version is listed as a variant. I haven't checked if every HP figure is featured, but I'd be surprised if not.
There are pages detailing creatures and other topics interspersed throughout the book: these cover Hogwarts uniforms; owls; snakes, frogs and dragons; skeletons; assorted other creatures; double sided heads and accessories.
There is a lot of detail in the book and it makes for fascinating reading. A lot of the facts quote which set a particular figure or accessory came in, but usually just its number is given. Of course we Brickset users have an excellent reference to look these up, but an appendix listing all the sets and when they were released would have been welcome. The text was checked by my AFOL friend Alastair Disley so we can be sure that it's accurate.
As I mentioned in a previous article, the book comes with an exclusive minifig, but unfortunately it's a bit boring: Harry in Slug Club Christmas party suit, and even more unfortunately, it's made in China. However if you have every HP fig (as I do) you'll need it for your collection which I think we can now consider to be complete given the chances of LEGO making more HP sets are slim.
The timing of the release of the book is interesting in that Harry Potter LEGO is effectively 'dead', or at best 'dormant', so it's not going to encourage sales of sets, but at the same time it's good that it has been released now and not, say, a year ago, because it is complete and won't be obsolete any time soon.
Following on from my involvement in the SW character encyclopedia, DK commissioned me to provide many of the images in the book. I haven't been through and counted but I estimate something like half of them were taken by me, including most of those on the 'creatures and other topics' pages. Consequently they look excellent :-)
The book really is a must for all Harry Potter LEGO fans and given that it's so cheap at the moment ($11 and £7.50 from Amazon) there is no good reason not to buy it.
Here are some sample spreads, posted on flickr:
Finally, here are the links to buy it at Amazon, and in doing so, you help Brickset stay on-air!