These are a handful of titles that should grace every book reviewer’s shelf. Our copies are dog-eared, filled with Post-it notes, and consulted again and again.
From Cover to Cover: Evaluating and Reviewing Children’s Books (revised edition) by Kathleen T. 
Horning (Harper, 2010).
“THE” book on evaluation. An absolute must-read for anyone selecting, recommending, or reviewing children’s books.
Picture This: How Pictures Work by Molly Bang (Chronicle, 2000).
The best explanation of how visual elements are used and combined to create meaning. Not only useful for understanding art in picture books, but a great primer on visual literacy.
A Family of Readers: The Book Lover’s Guide to Children’s and Young Adult Literature edited by Roger Sutton and Martha Parravano (Candlewick, 2010). 
Though directed at parents, this collection of essays and bibliographies is an excellent resource for anyone who thinks deeply about what makes a great children’s or YA book. It is particularly helpful for understanding how children progress from pre-readers, to emerging readers, to independent readers–and the types of books that are ideal at each stage.
Show and Tell: Exploring the Fine Art of Children’s Book Illustration by Dilys Evans (Chronicle, 2008).
A visually stunning deep dive into contemporary children’s book illustration. Using examples of work by David Shannon, Hilary Knight, Brian Selznick, Doreen Rappaport, and others, Evans highlights how each illustrator employs visual elements.