I just came across a fabulous resource for singers who want to improve their sightreading. (Or for sight-singing teachers looking for a good, inexpensive text.)
Ben Crowell, a professor of physics and astronomy at Fullerton College, put together a wonderful graded sight-singing book and has made it available for free on his website. (The work is released under a creative-commons license.) Printed copies are also available for purchase at Lulu.com. At present, they are less than $8 per copy.
Find more information and download it here: Eyes and Ears: A Book of Music for Sight-Singing
