Well, I stumbled across a remarkable text that the University of Chicago has put online in, well, what appears to be its entirety. In addition to links which lead to both general and city maps of the Indian subcontinent circa 1909, there's a massive Gazatteer of India. You can find Volume 1 here.
This in itself isn't all that remarkable. It is just an overview of India, climate, botony, zoology, ethnology, population and so on. However, what stuns me is that this is only volume 1 of 24.
Volume 2 launches into an historical overview, including numismatics (which ought to keep you busy for a little while), prehistoric antiques, pre-European literature and a nice section on Early European settlements.
Volume 3 is all trade and commerce. I don't want to dig extensively into it, but there are some nice tables and data going back to the 1860s. I am under the opinion that for D&D, statistical accuracy is not so important as getting data as close to the medieval period as possible. Backcountry India in the mid-nineteenth century was virtually indistinguishable - technologically - from the medieval period.
Volume 4 ought to be of interest to everyone, whether or not you have any intention of running anything like the real world. For example, this section on land use:
"Hedges and walls are, as a rule, unknown; at harvest time the waving fields of rice, wheat, or other food-grains reach almost without interruption to the horizon; and in intervals between the harvests an equally uninterrupted stretch of shimmering heat-baked soil fades away into the haze. Every morning during the cultivation season the Indian peasant - the most frugal and patient in the world - goes out to his fields with his cattle to work his well or to plough his land: and every evening he returns to his thatched cottage by the trees or his mud-built house in the closely packed village. His wants, such as they are, are met by the local artisans and menials, whom he rewards with customary contributions from his harvest."How sweet is that? I love these very old books, since the description borders on poetic. The above paints as nice an image as any DM could want, one which easily translates to a hundred worlds and a hundred imagined races - always giving, of course, a stronger sense of the world that the characters find themselves in, and which the DM must hold in mind if the world is to feel real.
There's 38 pages on land revenue, 29 pages on local and municipal government, 58 pages on the army, 14 pages on currency and banking ... all this in just volume 4.
Volumes 5 through 24 is the core of the Gazatteer, with individual entries on hundreds and hundreds of regions, cities and villages, as well as geographical features, all written in the aforementioned style and with emphasis on strictly geographical and historical features. It, too, tends to be poetical. I leave you with a description of the battle at Aliwal (which I cannot improve upon). I stumbled across it as one of the pages in the gazatteer. There's tons and tons of stuff, of every kind:
"The Sikh force, which amounted to about 15,000 men, was posted in the lowlands close to the Sutlej [river], with the right resting on the village of Bhundri on the high bank, and the left on Aliwal close to the river. East of Bhundri the high bank or ridge, which separates the valley of the Sutlej from the uplands, sweeps inwards in a semicircle to the distance of 5 or 6 miles, crowned with villages at intervals, and leaving a wide open plain between it and the river. It was across this plain that the British army on the morning of January 28  moved to the attack, the capture of the village of Aliwal, the key of the position, being the first object. The Sikh guns were as usual well served; but Aliwal was in the hands of inferior troops, and the resistance was spiritless. By the capture of the village the Sikh left was turned; but round Bhundri their right, composed of enthusiastic Khalsa troops (trained by Europeans), made a most determined stand, and the whole battle is still called by the natives the fight of Bhundri. The most gallant part of the action was the charge of the 16th Lancers on the unbroken Sikh infantry, who received them in squares. Three times the Sikhs were ridden over, but they reformed at once on each occasion; and it was not till the whole strength of the British was brought to bear on them that they were at length compelled to turn their backs. The Sikh troops were either driven across the river, in which many of them were drowned, or dispersed themselves over the uplands. The British losses were considerable, amounting to 400 men killed and wounded. A tall monument, erected in the centre of the plain to the memory of those who fell, marks the scene of the action."