Buxton, B.H. The Buxton Encyclopedia of Watermarks. Tappan, N.Y.: Buxton Stamp Co., 1977 114p.
Felix, Ervin J. The Stamp Collector's Guidebook of Worldwide Watermarks and Perforations, from 1840 to date. Racine, WI.: Whitman Publishing Co., 1966 256p.
Repeta, Louis E. Watermarks In Postage Stamp Paper: a comprehensive look at a key stamp element. Reprinted in 1999 from The American Philatelist (February 1987). 27p.