I have recently become a big fan of Brain Pickings, and while poking around, I found this awesome article from January: New Year's Resolution Reading List: 9 Books on Reading and Writing
I love these book suggestions. I've read The Elements of Style (though not Maira Kalman's illustrated version - that would have been much more interesting), Bird by Bird and Zen in the Art of Writing, and they were great. While reading the last two, I would suddenly have urges to throw the books down and run off to write. That's a rare feeling to come over me from reading, but it happened, and it's a testament to not only how useful the books are, but how inspiring. They make you feel like you really can create something of meaning, value and beauty, and you just want to run off and do it!
I would also recommend Writing Down the Bones by Natalie Goldberg. Great stuff.
What's your favorite book on writing?