Hey Matt -
I saved this bit when I checked into a writing job for textbooks. (no, I didn't apply).
It was from Elance - hope they don't mind my sharing it with you.
The following are sources we use that may be of help to you, although the first two are currently out of print. We also use, but do not rely solely upon, Spache (Grades K-2), Dale-Chall (Grades 4-12), and Fry (all grades) readability formulas.
Dale, Edgar, and Joseph O’Rourke. The Living Word Vocabulary. Chicago: World Book--Childcraft International, 1981
Harris, Albert J. and Milton D. Jacobson. Basic Reading Vocabularies. New York: Macmillan Publishing Co., 1982
Taylor, Stanford E., et. al. EDL Core Vocabularies in Reading, Mathematics,
Science, and Social Studies. Columbia, SC: Educational Development Laboratories, Inc.,
1989
Mogilner, Alijandra. Children’s Writer’s Word Book. Cincinnati, OH: Writer’s Digest
Books, 1992
Also, the Flesch-Kincaid Grade Level can be found under “Readability Statistics” in Microsoft Word under “Tools”, then “Spelling and Grammar.” Click through all the suggested corrections. At the end the “Readability