Bottom line, though, is that librarians also need to be able to apply evaluative criteria to books, to make some determination about selection, to hone their skills in evaluating books themselves. Different genres, forms, and formats have some standard criteria (qualities of plot, character, setting, etc.) but there are other considerations as we move from GNs to picture books to serial fiction to nonfiction of all sorts.
The deliberations also go beyond the use of evaluative criteria. Will this be a book that will have legs? That will have lasting appeal? Or is it a book that will be popular for a short time and then fade? Is it a book that is developmentally appropriate? When budgets are pushed to the limits, sometimes librarians need to make tough decisions. I know of districts where the annual budget for books per school is $3000 (and my accountant can attest that there are years that my own budget exceeds this figure). That might buy 150-200 trade books or even fewer reference tools or subscriptions to data bases. Being careful, then, narrowing to the best choices, is crucial.
Being able to differentiate between a "good" book (one I like) and a "good" book (one that meets evaluative criteria and meets needs of readers within the school) is of value not only to school librarians but to educators and parents as well. I know when I enter the chain book stores (which is all we have locally) and see the displays, they are not always those "good" books. I see parents trying to decide which book to purchase when there are so many from which to select. There needs to be someone who can help narrow down the selection a bit until kids and parents and teachers feel comfortable of their own ability to select that next best book.