password aging

[Adam Shostack <adam () weathership homeport org>]

        Various people assert that its a good idea to maintain a
history of user passwords so that they can't change their password to
a previous password.  However, I'm having trouble finding a reference
to this in the literature that examines the issue of how many
passwords to save and why.  The lime green book (password management)
says not to let the user use their previous password, but doesn't go
into storing a history.

        Does anyone know of a paper on, or that discusses, this topic, 
and how or why to pick various values of N?

Adam