Publishers
Some publishers produce reprints of older books relating to India, or books in respect of specialised topics such as military.
Its website says that its range of books covers the whole spectrum of military history with titles on uniforms, battles, official and regimental histories, specialist works containing medal rolls and casualties lists as well as titles for genealogists, medal collectors and researchers.
An example of one of its books is "The East India Company's Arsenals & Manufactories" by Brig-Gen Henry A. Young, Director of Ordnance Factories in India 1917-1920, first published in 1937 and re-published by Naval & Military Press in 2005 which gives historical background of munitions factories etc.
Also see Military reading list.
Leonaur
Its' website says Leonaur is dedicated to publishing hard to find and rare books at prices that are no greater than those of popular, mass market titles. Its books on Military History include titles relating to India.
An example is Galloping guns : the experiences of an officer of the Bengal Horse Artillery during the Second Maratha War 1804-05 by James Young which is described in this link
Pagoda Tree Press
Its' website says Pagoda Tree Press has been developed to publish a range of new books on the History of India, Tibet and the Himalayas; with a particular emphasis on the History of Photography in that region; and also to republish a variety of rare and unusual historical works on that region; early Guide books, Maps & Pamphlets, useful historical & reference works, together with long out of print travel writing from the Himalayan region.
An example is Residents of Simla 1898 being an Alphabetical List of the Principal British Residents and their addresses.
Antiquarian books on India are also sold through the associated website http://www.indiabooks.co.uk