M. Hofferber Books
Specializing in nature, science, how-to, guidebooks, cookbooks and western Americana.
http://www.webcom.com/outbooks/

Mackus Company
Acquisition and sale of illuminated manuscripts and historical legal documents. Statement of authenticity included.
http://www.mackusco.com/

Madden Books
Rare, used antique and out of print books, specializing in politics and history including Chinese, Cuban, Soviet and Socialist publications.
http://www.maddenbooks.com/

Maggs Rare Books
Multi-speciality rare and antiquarian booksellers.
http://www.maggs.com/

Maremagnum Librorum
Antique, old and out of print books; in English, French, German,Italian and Spanish.
http://www.maremagnum.com/

Maxwell's Bookmark
Rare and out of print books, including California, Scottish history, genealogy, illustrated books.
http://www.maxwellsbookmark.com/

Meetinghouse Books
Old, used and out-of-print bookshop; includes world history, literature, cultural studies, arts, and natural history.
http://www.meetinghousebooks.com/

MemoraDelia
Used science fiction, fantasy and horror books. Plus collectibles, independent press.
http://www.memoradelia.com/

Michael Ginsberg Books
Western Americana, American History, Canadiana, Voyages and Travel, Bibliography and Reference books.
http://www.ginsbook.com/

Michael Treloar Antiquarian Booksellers
Particular enthusiasms are modern first editions, cricket ephemera, and North Australiana.
http://www.treloars.com/

Mitzi Witch's Used Books
Fiction and non-fiction in a variety of areas.
http://usedbooks.freehosting.net/

Mockingbird Books
Used bookstore with used, rare, out-of-print, and hard-to-find books in all subject areas.
http://pw2.netcom.com/~mkbird/

ModernRare
Rare, out-of-print, and signed books for collectors, specializing in modern first editions, photography, and one-of-a-kind books.
http://www.modernrare.com/index.cfm

Moe's Books
Catalogs of out-of-print, new, and antiquarian books in all subject areas updated weekly.
http://www.moesbooks.com/

MojoMax Books
Rare used books; including classical literature, art, history, Americana, and modern first editions.
http://www.mojomax.com/index.php

Motte And Bailey Booksellers
Specializes in history and science fiction works, variety of general stock as well.
http://www.mottebooks.com

Mr. Kolk Raamatuantikvariaat
Rare and old books from Estonia. Maps, postcards and prints also available.
http://www.antikvariaat.ee

Mumfordbooks
Secondhand rare and old books; first editions, out-of-print.
http://www.mumfordbooks.co.uk/

Mystical Unicorn
On-line shopping for used books, bibliographies and author information.
http://www.myunicorn.com/