"They Say/ I Say" by Gerald Graff & Cathy Birkenstein (W.W.Norton & Co., 2006, 1st edition) 176 pages
-- A terrific little book to improve your essays, highly recommended for high school-college level.
"Writing Without Teachers" by Peter Elbow (O.U.P, 1996) 200 pages
--- Excellent, innovative writing exercises - already a minor classic
" Clean, Well-Lighted Sentences" by Janis Bell (W.W.Norton & Co., 2008, 1st edition) 140 pages $14
--- For intermediate students & business people - all the basics, very well-presented, with some exercises