Some journals publish reviews of newly published books in their field. It's not clear how authors for these reviews are typically chosen. Can you apply for writing a review? Or should you do just like for a regular paper: buy the book, read it, write your review and submit it? How is this typically handled? Is it reserved to the editors to initiate such reviews?
I am sure this varies but is most likely one of the following (in some kind of falling order):
you are a well-known and respected researcher/educator in the field
you have recently written material that are key to the book (perhaps referenced in it)
you are known not to know the word "No" (makes the editors work easier)
you are friends with the editor (makes his/her life easy)
I doubt many editors will necessarily take you up on an offer (depending which category you may fall into) but I am sure some might be fine with it. It never hurts to contact an editor and express interest. You should then provide a background for why you should be considered for such a task.