There are some short but informative videos in a recently created series on YouTube called Fridays with Agent Kristin. Some of videos helped me better understand the common differences between YA and middle-grade fiction, especially this one:
I think my current novel-in-progress, Moonrise Ink, falls within the upper-level-middle-grade arena.
Also a great writing resource is the agent’s blog, Pub Rants.
Over on Write About Dragons you can find a great number of video lectures from the creative writing classes of the awesome fantasy author Brandon Sanderson. In the lectures, he talks about creating characters, plotting, writing description, and more, as well as some insights into how the book business works. I have found these lectures to be extremely helpful and informative; they’re definitely worth checking out.