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

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/

I.C.Books  
   Used and rare books, sorted by category.
   http://icbooks.homestead.com/

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

Ian Barfield Books.  
   Out of print and second hand academic books; philosophy a speciality.
   http://www.ianbarfield.co.uk/

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/

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