is close to worthless since Back of the Book precancels of what I've got rarely if ever show up.
And I have 2 older publications of the precancel society:
-- The Precancel Stamp Society's Town and Type Catalog of the United States and Territories, 7th edition, 2007
-- Precancel Stamp Society Catalog of United States Bureau Precancels, 5th edition, 2005
Except for ID'ing precancel types I don't think they are much good for pricing.
What I've been thinking is to price the stamp from Scott and add a little bit for the precancel.
For selling purposes auction them starting about 20% of that calculated value.