What reference books do you find most useful? As I continue to downsize and declutter I am looking at my library.

These books I use the most.
Way Beyond Monochrome by Lambrecht and Woodhouse
Beyond the Zone System by Phil Davis
The Darkroom Cookbook by Steve Anchell
The Ansel Adams Guide – Basic Techniques of Photography by John Schaefer
Using the View Camera by Steve Simmons

What other books would you add to the bookshelf?