An interesting, and apparently useful, thread on writing book reviews has some interesting information.
As a beginner, there are normally publications (web based and/or print) associated with professional socieities or the like that frequently publish a list of “books recieved.”
You write a letter of inquiry to the review editor. State your qualifications (I’m interested in reviewing XYZ because [how it fits your work]. I have [relevant academic qualification]). Once you get more pubs, list a few major/relevant ones.
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If you establish a reputation for: a. meeting (not exceeding) word counts; b. timely delivery; c. balanced review (alas, often in that order), you’ll get more work than you want.
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Also, make sure the books are tightly relevant to your area of concentration. You don’t want to waste effort or become too diffuse (or have, in time, your evaluations lack credibility; that you look like a review-hack).
From John Proctor
For me, this is again an issue. What is my field of interest? How much specialization do I need to have? How should I decide on this? I guess I need to think on this some more.
And John Proctor came back and weighed in on what a good review should look like, a very useful set of information.
By my lights, a good review should:
1. be a reasonable, stand alone, readable document. It should have interest as a discrete piece of writing.
2. Focus on the contents of the book (thesis, data, methodology, key insights resulting).
3. Be a critical but fair evaluation (reading the book on the book’s own terms. Note lacunae in data, consistency in methodology, validity in thesis).
4. Address potential audience clearly. At minimum, this should be “this book would be an excellent survey of X for students of Y” or “this book is best aimed at graduate students or professional scholars beginning a study of X,” or “this book advances the scholarly conversation on Y.” Best is to not just tell me who it’s best for, but why “this book would be a ready supplement to the undergraduate classroom because [specific reasons].”
In other words, it should indicate clearly who should read this book and why.
Finally, there’s 5. point to what the field/scholar/reader of the book should do next. “X is a fine treatment of Y; however, Z remains…” This latter will be, true, of most use to those others who have read the book.
And you’ll have between 300 to 1500 words to accomplish all this.
Writing a good review is not easy; it’s an scholarly artform of its own (perhaps, as a writing exercise, as difficult, if not more so, than a uniquely composed peer-reviewed essay).
I don’t think I would have as much trouble writing a book review, but I probably need to decide on a few areas of interest and push those.
JerseyJay has a very different perspective, but it still seems useful:
For what it’s worth, this is my method:
1. I choose a broad research topic that interests me and I research it with an eye towards publishing articles/monograph about it.
2. I keep my eye open for relevant new books that would be useful.
3. I find one that I think could be useful and that I don’t want to buy.
4. I write to a relevant journal with a resume of my interests and CV and ask if I can do a review.
5. If they say yes, they send me the book and I read it, use it for my broader research, and write a review.
6. If not, I try at another journal.
Thus, rather than a diversion from my research, book reviews serve as building blocks for it.
Very rarely have I been turned down for a review. If so, it is because they have some protocol for selecting reviewers or the book is already assigned to some other reviewer.
I think I need to think about this.
One problem with review article, again by John Proctor, but on a different thread:
I think review articles become a demerit if:
1. they are “all over the place” in terms of speciality. That makes it look like you’re just doing hack work for the sake of getting a by-line in print somewhere and not seriously pursuing a scholarly niche.
This is an issue for me. Where am I looking? Gotta go back to MindMeister and see where I am and where I want to be.
However, I have checked in and I am still scheduled for four classes. I expect that, unless there is an influx of students, one of the classes will not make and the students from the two sections of one course will be combined into one. Right now there are 9 and 10 in those two classes. 
quote from Terry Pratchett’s Wee Free Men: