Ironically, I sorted and cleaned my Mustang Reference shelves this weekend. Put all the loose articles in document protectors, put them in notebooks, and put their titles in Excel and made a table of contents.
Here are the big users:
Ford Shop Manual for each year
The Big Chilton's manuals, about 3" thick, bracketing the years I have. For example, for my 1970 Mach, I have a 1969, 1970 and 1975. Since the books have the original part numbers, and cover about 10 years each, I can find part numbers that fit the Mach, say on a 1974 Mercury.
Mustang Monthly, have a full set, all the way back to the first 2 issues that were not called Mustang Monthly, in binders (about 7 feet of shelf space)
Mustang Does It
Mustang Recognition Guide
How to Rebuilt your Ford Engine
Ford High Performance Parts Guide
and about 30 more books, etc., and scores of articles, links, etc.
I also have about 10 vendor catalogs for various pictures, etc.