"One would think that it would be easy to find some useful published discussions of recommendation letters, but there seems to be very little written to guide faculty members in this difficult and crucially important task..."

Here are some other useful resources I've found on this subject.

Chapnick, Adam. "The ubiquitous reference letter." University Affairs 15 Jan. 2007 http://www.universityaffairs.ca/the-ubiquitous-reference-letter.aspx

Wandless, William H. "The academic job market: letters of wreck." Otherwise, Lightning [blog] 12 Oct. 2011 http://williamhwandless.wordpress.com/2011/10/12/the-academic-job-market-letters-of-wreck/

A lot of this one is impressionistic (and a bit cynical sounding), but the fourth section has more solid and practical advice.