As a student I have found academic book reviews to be a really helpful resource. Academic book reviews are just what they sound like: reviews of academic books often found in academic journals. Academic books go through a peer review process before they are published and hit the shelves to help ensure their quality, but book reviews provide an opportunity for the academic community to begin publicly assessing these works.
As I student, I have found book reviews to be useful in a couple ways:
(1) As companions to a book you're working through. By reading reviews of a book you're engaging you can get some insights into how the work has been received, and can also find some helpful analysis and critiques of the argument. In working on my thesis I interacted with a lot of Dennis Macdonald's work on the influence of Homer on the New Testament and other early Jewish and Christian texts. Book reviews (and MacDonald's response to some of those reviews) were helpful for me, as they alerted me to some of the aspects of MacDonald's work that had not been well received by some scholars. Since I was making positive use of some of MacDonald's work this allowed me to address some of the criticisms of his work and draw distinctions between what he had argued and the argument I was making.
I have found book reviews to be especially helpful when I am working through a book that is outside of my field. When you're reading within your field you are better able to critically engage a book. Sometimes when a work is outside your field you don't really have the background information to weigh the merits of an author's arguments. And in some cases, you don't have the time to familiarize yourself with a wide range of literature and opinions in that field. In this case book reviews can alert you to different perspectives on the topic or potential weaknesses in the author's argument (although it seems some reviewers are sheepish about offering criticisms). They can help you from being unduly influenced by a single work.
(2) As a way of taking the pulse of scholarship and staying abreast of developments. Book reviews are nice and short. Reading them is a great way to stay familiar with what is being published, what kinds of arguments are being made and what kind of work is being done. I find this can be really helpful when it comes to cognate disciplines, for which you don't have the time to read lots of books, but you'd still like to keep an eye on what's going on.
(3) Research paper ideas. When you're trying to develop a topic for a research paper book reviews can be a helpful source of inspiration.
(4) Finding stuff to read! One of the big reasons book reviews of all kinds exist is to help people decide what to read. With so many books and so little time book reviews can help you sift through the many options and decide which books to add to your list.
How to find them: You can look through print journals in your library. But the best way to find them is to use your library's full text databases. Your librarian can help you find the right database for what you're looking for. The databases I use give you the option to limit your search results to book reviews. If you have a particular book you'd like to read reviews on, you can just search for the title. If you're looking for reviews on a particular subject, you can enter your search terms and narrow the results to reviews. So, I am currently doing some research on the Book of Jude: I simply typed "Jude" in the search field and narrowed the results to book reviews.
For the areas this website focuses on, two great, free, online sources for book reviews are the Society of Biblical Literature's Review of Biblical Literature and the Bryn Mawr Classical Review.