Sentence-by-sentence writing improvement:

• Line by Line: How to Edit Your Own Writing, Claire Kehrwald Cook,Houghton Mifflin
• A Dictionary of Modern English Usage, William Fowler, Oxford Univ. Press
• The Careful Writer, Theodore M. Bernstein, Atheneum
• The Practical Stylist, Sheridan Baker, Harper & Row
• The Transitive Vampire: A Handbook of Grammar for the Innocent, the Eager, and the Doomed, Karen Elizabeth Gordon, Times Books (also The Well-Tempered Sentence by her)
• The Elements of Style, Strunk & White

Writing better fiction:

• Beginnings, Middles, and Ends, Nancy Kress, Writer’s Digest Books
• Creating Short Fiction, Damon Knight, St. Martin’s
• On Becoming a Novelist, John Gardner, Harper & Row
• The Art of Creative Writing, Lajos Egri (may be OOP)
• Techniques of the Selling Writer, Dwight V. Swain
• Science Fiction Writers Workshop I: An Introduction to Fiction Mechanics, Barry B. Longyear
• How to Write a Damn Good Novel II,James N. Frey. St. Martins 1994, ISBN 0-312-10478-2
• Characters and Viewpoint, Orson Scott Card

Writing better speculative fiction:

• The Craft of Science Fiction, Reginald Bretnor
• Those Who Can, ed. Robin Scott Wilson (recently reprinted)
• Paragons, ed. Robin Scott Wilson
• How to Write Science Fiction and Fantasy, Orson Scott Card, Writer’s Digest Books
• Writing Science Fiction and Fantasy, editors of Analog and Asimov’s
• Aliens and Alien Societies, ed. Ben Bova, Writer’s Digest Books
• Creating the Heavens, Melissa Scott, Heinemann

Style manuals (cover grammar, punctuation, usage, treatment of names and terms, editing stages, book production):

• The Chicago Manual of Style, University of Chicago
• Words into Type, Prentice Hall
• A Manual of Style (U.S. Govt Printing Office), Gramercy
• The New York Times Manual of Style and Usage