The Artist’s Way by Julia Cameron
I never thought this kind of spiritual book would be for me, but I was wrong. My biggest take-away has been the concept of morning pages. I've done them consistently for over four years and I amazed at how much they've improved my writing and my general creativity. Some days my pages are just ho-hum to do lists or dream journaling, but other days it's more. For example, I came up with the idea for my second novel and wrote most of its first draft in my morning pages.
Mary Kole's Kidlit.com and Writing Irresistible Kidlit
Mary Kole is a former agent and current freelance editor. Her blog is a fount of information and her book, Writing Irresistible Kidlit was one of the first craft-focused texts I've ever read (and one of the best). I've worked with her before, and her editorial feedback was encouraging, deep, and thoughtful.
Joining SCBWI is one of the best things I did when I was first starting out. Writing can feel like such a solitary endeavor, which is why SCBWI's vibrant kidlit community is so amazing. It hosts both local and regional events, conferences and get-togethers, as well as contests, grants, volunteer opportunities, and so much more.
I'm a big podcast person, and there are many excellent kidlit-focused ones out there. My favorites include First Draft with Sarah Enni, ABLA agent Jennifer Laughran's Literaticast, The Narrative Breakdown (they don't really post anymore, but their archives are still full of great episodes), School Library Journal's The Yarn, 88 Cups of Tea with Yin Chang, Print Run Podcast, Nina Lacour’s Keeping a Notebook, and the SCBWI podcasts, which are available here if you're a member.
It can feel harder to write a query and/or a synopsis than it is to write a full novel. Thankfully there are resources to help, including QueryShark, which is run by agent extraordinaire Janet Reid. Writers submit their query letters and Janet (aka The Shark) shreds them to pieces. There's a huge archive, so you can dig around until you find query letters for projects similar to yours.
Another great resource, with author spotlights, editor and agent interviews, giveaways, and a weekly newsletter, The Weekly 411, which I look forward to reading each week.