What is a WC, you ask? (Hint: It's not a potty in France.) The publishing biz loves abbreviations. A few years ago Dahlia Adler shared a fantastic glossary to help you wade through the alphabet soup of MSs, LIs, and CPs.
Literary agent Janet Reid hosts this must-read resource for querying writers. Though new-comers should read the entire website, in the post linked above, the shark herself boils down the answer to the essential question: What is a query letter?
Literary agent Jennifer Laughran keeps her former blog open for references like these, and Wordcount Dracula is one of the most popular posts. In it she explains the accepted norms for word counts (WC) in children's books.
Literary agent John Cusick addresses taking measure of yourself as a writer. He also recently posted here about effective queries.