Iain Grahame Rare Books
Specialist in rare natural history books, shooting books, stalking books, hunting books, fishing books, bird books and africana books and prints.
http://www.iaingrahamerarebooks.com/

Ice House Books
Second-hand and antiquarian books and publications; special interests include socialist, marxist and progressive fiction and non-fiction in English and other languages.
http://www.icehousebooks.co.uk/

Inlibris
Fine and rare books; specializes in autographs, manuscripts, and archives of significant personalities of science and the arts.
http://www.rarebooksandautographs.com/

Isis Antiquarian Books
Specialises in Philosophy, with books about art, history, religion and literature. Bilingual English and Dutch.
http://www.isis-books.nl/english/english.htm

The Ink Company
Rare and out of print books, sorted by specialty and searchable.
http://www.inkcobooks.com/