Hi Mike,
As somebody who sometimes writes book reviews (I have a fresh stack on my desk at the moment!) I understand how daunting it can get! To help beat the dreaded "writer's block" I tend to stick to a basic outline or formula to make sure I cover all the bases, and return to it several times during the process rather than try to write the whole review in one go. And I do only one at a time to avoid getting mixed up with multiple files... (Nothing more embarrassing than putting info into the wrong review! Been there, done that.)
First up is an intro to the subject, for those who may not know much about it. This should be only a paragraph or two (at most), and if possible it should point out how the new book fills a gap or adds something new to the subject.
Then you need to list some basic details (title, author, publisher, number of pages, number of photos, does it have footnotes and a bibliography, etc) to tell the would-be customer what they're getting between the covers for their money. Personally I like to see reference books with at least a bibliography and index, and appreciate it if reviewers warn me about it before I buy. Some folks will list the chapter headings from the Table of Contents too.
With that boring stuff out of the way, now you can add a paragraph or three about the contents and how well you think the author did! Look for accuracy (is it based on well-known facts? and does he get them right?), good development of arguments (does he actually cover the subject as advertised on the cover?), and of course decent editing (the existence of spelling mistakes & bad grammer should be pointed out if they are frequent enough to interfere with reading). I also critique the photos regarding whether they are clear or grainy, big or tiny, etc because modellers want to know if they are useful as reference. A note about the quality of captions is good too.
Round it off with a conclusion assessing whether the book is any good, and who might benefit most from it. This can be tricky because you don't want to be unduly harsh just because it didn't offer what you personally were looking for; often a good history book may not be good for photos, while a nice picture-book may not explain the action, so each book will have target audiences to whom you can recommend it. (Unless it's just a stinker, in which case remember your only protection from libel suits is your ability to back up your printed opinions with examples from the book...) Fortunately my experience of Stackpole is mostly positive!
Have a look around some of the reviews here on Armorama to get a flavour.
Hope this helps,
Tom
