Caplan, H. H. The Encyclopedia of Artist's Signatures, Symbols and Monograms. (1999)
Avery Reference N45 C173

Castagno, John. Abstract artists: signatures and monograms, an international directory. (2007)
Avery Reference N45 C269
(Note: Castagno publishes many other signature directories—some are listed below)

Castagno, John. African, Asian, and Middle Eastern artists: signatures and monograms from 1800: a directory. (2009)
Avery Reference N45 C2695

Castagno, John. American artists I-III: signatures and monograms [from 1800]. (1990-2009)
Avery Reference N45 C27, N45 C271, N45 C2711

Castagno, John. Artists monograms and indiscernible signatures I-III: an international directory, 1800-1991. (1991-2009) 
Avery Reference N45 C2725, N45 C27251, N45 C272511

Castagno, John. European Artists Signatures and Monograms 1800-1900. (1990)
Avery Reference N45 C273

Castagno, John. Jewish artists: signatures and monograms: an international directory. (2010)
Avery-LC N45 .C384 2010

Castagno, John. Latin American artists’ signatures and monograms: colonial era to 1996. (1997)
Avery Reference N45 C2735

Castagno, John. Old masters: signatures and monograms I-II. (1996-2009)
Avery Reference N45 C2734, N45 C27343

Jackson, Radway. The Visual index of artists’ signatures and monograms. (1991)
Avery Reference N45 J13

Lampe, Louis.  Signatures et monogrammes des peintres de toutes les écoles et époques. (2 vol.)  (1999?)
Avery Reference N45 L19

Pfisterer, Paul. Signaturenlexikon = Directory of signatures. (1999)
Avery Reference N45 P483

Self, James. Japanese Art Signatures: a Handbook and Practical Guide. (1987)
Avery Reference AK 3640 Se37

Signatures & monogrammes d’artistes des XIXe et XXe siècles. (1999)
Avery Reference N45 Si26