I've noticed that there are some great resources out there, especially for teaching, but that learning about them is hit or miss for me. So I'll share a couple, and hope some others will share and teach me new ones.
For lit folks: the MLA Approaches to Teaching series. Basically, an editor puts together a group of essays by different people about how to teach some specific text or group of texts. These are especially helpful to me when I'm teaching a text for the first time. The best part is that there are usually some overview type essays and essays specifically aimed at teaching texts at different levels, from high school to graduate school.
For Shakespeare: Russ McDonald's Bedford Companion to Shakespeare. I have my Shakespeare students and and early modern lit students read it for background about the period, social and theatrical practices, and so on. It's very readable, with interesting documents from the period that you can use in a variety of ways.
For Chaucer: Helen Cooper's Oxford Guides to Chaucer: The Canterbury Tales. She's organized the text through the tales, primarily, which means I use it to review as I reread the tale and think about what I'm doing with it in class. (I think I have the second edition, but I'd adore a new one.)
For composition: Gerald Graff and Cathy Birkenstein's They Say / I Say. I use it in my comp classes, and it does a really fine job helping students understand how to use other people's words and ideas respectfully and well to make their own arguments.
What I'd love: some really good suggestions for introductory theory texts and history background stuff. And Shakespeare, Chaucer, and comp resources that are really helpful.