Most recently I've read Les Edgerton's Hooked and Noah Lukeman's The First Five Pages: A Writer's Guide to Staying out of the Rejection Pile. I got a lot out of both, though if I had to choose, I'd recommend The First Five Pages, because (as the title suggests) it focuses on that all-important start to your novel.
Over the last three years I've read dozens of writing books. Some proved more helpful than others, though I suspect much of that has been due to where I was as a writer, rather than any failure on the book's part. They all contain useful information, often not discovered on the first read (I think re-reading books on writing is a must). Some I come back to time and again, like the excellent Writing the Breakout Novel, by Donald Maass, and Blythe Camenson & Marshall J. Cook's Give 'Em What They Want, (which has the best instructions and step-by-step guide to producing a synopsis I've found to date).
At the last GSHW meeting, Elizabeth Bear (matociquala) recommended Techniques of the Selling Writer , by Dwight V. Swain, and Story, by Robert McKee. I haven't got around to reading them yet, but they're next on my non-fiction list.
I'm sure there are some other great books out there. What was the last one you read? Which ones have you found most helpful, and why?