Plain text ebooks from the authors and publishers at Smashwords.  They lack much formatting, but they're readable on virtually anything.

	Self-Editing: Two Half Brains Make a Whole Writer	by Deborah J. Lightfoot Sep. 20, 2011	$0.99	13079 words	Sample 30%
	Semicolons and Colons: A Guide for the 21st Century	by Lenny Everson Aug. 28, 2011	Free!	2834 words	Read a sample
	Reading English News on the Internet: A Guide to Connectors, Verbs, Expressions, and Vocabulary for the ESL Student	by David Petersen July 08, 2011	$7.99	27987 words	Sample 20%
	The Passive Activist	by Graham Murray June 30, 2011	$0.99	2096 words
	Come Again?	by Graham Murray June 24, 2011	$0.99	6171 words	Sample 30%
	Guia Prático da Reforma Ortográfica	by Walace Freitas May 15, 2011	$0.99	21981 words	Sample 20%
	Book on English Grammar, Punctuation and Capitalization.	by The Gifted Learning Project April 15, 2011	$0.99	12788 words	Sample 3%
	300 Days of Better Writing	by David Bowman July 07, 2010	$9.95	64371 words	Sample 20%
	Common Writing Errors Workbook	by Katherine Ploeger July 01, 2010	$14.95	33988 words	Sample 20%
	Writing Clear Prose	by George Perkins April 28, 2010	$2.99	28592 words	Sample 30%