If you're serious about improving your writing craft, read each of the following four books:
The Curious Case of the Misplaced Modifier by Bonnie Trenga
One of the most witty and useful books you'll ever find on common grammar and style errors.
Effective Writing by Bruce Ross-Larson
Rhetorical techniques, phrasing ideas and other verbal flourishes you can use to punch up your writing and convey your message with flair.
The Elements of Style by William Strunk and E.B. White
I've read and reread this tiny book several times over the course of my life, each time finding new and valuable ideas in it. A must-read for any serious writer.
Wild Mind by Natalie Goldberg
An immensely freeing book that gives you courage with every page. Contains an enormous collection of writing prompts (see for example the I don't remember prompt) to take your mind to new levels of creativity.