Textbooks
Kate Turabian's classic Manual for Writers of Term Papers, Theses, and Dissertations because it seems to help students with varying levels of problems. (That's U of Chicago Press, various editions — the most recent of which have a coauthor, but I don't remember who it is.)
Wolfram's Dialects and American English and Davis' English dialectology
Celia M. Millward, in her Biography of the English Language
John C. Wells' Accents of English, in two volumes.
Linguistics for Students of Literature by Elizabeth Closs Traugott and Mary Louise Pratt (1980)
Style in Fiction by Geoffrey N. Leech and Michael H. Short (1982)
Dialects and American English by Walt Wolfram (1991), Prentice Hall. Important news: this is out of print BUT is coming out in a new edition this year, co-authored (I believe) by Natalie Schilling-Estes.
James Paul Gee, An Introduction to Human Language: Fundamental Concepts in Linguistics Prentice Hall, 1993
Joseph C. Blumenthal, English 3200: A Programmed Course in Grammar and Usage New York: Harcourt Brace, 1994. It's essentially "school" grammar, and some of my students did not like programmed instruction, but anybody who ever worked through the program could not help but learn grammar and usage. This book was pioneered in the early sixties, was "desexed" in the eighties, and given "writing applications" in the fourth edition (1994). The grammar and usage which the program teaches is mostly the stuff that those of us who are legally old learned in elementary school but which today is only to be found in graduate linguistics courses for the gifted and talented. It's ideal for self-study and much, much cheaper than the computerized "programs" that teach English fundamentals. There are smaller versions entitled English 2400 and English 2600, but the program is essentially the same. The numbers refer to the number of frames [Bit of instruction] in the books. The same books come in both "school" and "college" editions, but the program is identical in each. Many of my colleagues didn't like English 3200 because it didn't leave them anything to do. However I found that it freed me to teach writing.
Origins and Development of the English Language, Thomas Pyles and John Algeo