If You Don't Send It They Won't Publish: Submission Advice from the Sub It Club
There's a wonderful website for authors and illustrators called the Sub It Club. In its pages you'll find a compendium of practical advice about the (often overwhelming) submission and (frankly terrifying) querying process. The site is loaded with scads of articles about agents, editors, query letters, pitching, social media, and even ways to cope with rejection.
The Sub It Club is curated by Heather Ayris Burnell, Dana Carey, Lisha Cauthen, and Amy Dixon.