Plate Number Coil Collectors Club
Stamp Community Family - Stamp Forum
Stamp Smarter - Philatelic Learning Center
The Swedish Tiger - US Stamps, Values, and Identification

United States

Alexander, Thomas J. - U.S. Postal Markings 1851-1861
Arfken, George B. - Postage Due - The United States Larger Numeral Postage Due Stamps 1879 - 1894
Brookman, Lester G. - The United States Postage Stamps of the 19th Century (3 volumes)
Eubanks, Gordon - Imperforate Issues 1851-1856
Griffith, Gary - United States Stamps 1922-1926
Griffith, Gary - United States Stamps 1927-1932
Johl, Max G. - US Postage Stamps of the 20th Century (4 volumes)
Mooz, William - The Special Printings of the 1879 Postage Due Stamp
Rose, Jon W. - First United States Perforated Stamps - The Issue of 1857

United States Plate Number Coils

Agris, Joseph - The Transportation Coils and other Plate Number Coil Issues
Denson, Ed - Plate Number Coils on First Day Covers, Souvenir Pages, and Ceremony Programs
Esrati, Stephen G. - Plate Number Coil Catalog 1989, Fifth Edition
Lawrence, Ken - Linn's Plate Number Coil Handboook
Nazar, Richard - The Plate Number Coil Catalog, Eighth Edition
Washburn, Robert M. - PMCs: A guide to Collecting Purple Machine Cancels