Books of Writing Advice

I swear someone (I'm pretty sure it was Shawn Merwin) posted something in my comments in the not too distant past recommending a couple of titles on the art of writing. It's become relevant to a project I'm working on now at the library, and I can't find it. Alas!

So, here's where you all come in: are there books on writing that you've found particularly useful? This could be for nonfiction, fiction, poetry, screenwriting, comics you name it. I just want to make sure the titles I'm recommending are quality! I've already pulled the wonderful list from Wizards of the Coast, but would love further suggestions.

Thanks!