Travelling in India has definitely been made easier by the emergence of comprehensive guidebooks aimed at the backpacker and the mid-range tourist. Old time travelers, who roamed the world before the advent of guidebooks, relied on each other for information and bemoan the lack of camaraderie among today's travelers.
It is true that guidebooks tend to channel tourists toward the same places, send them to the same hotels, and create a "check list" approach to sight seeing. However, the comfort of having reliable travel information and advice gives a lot of people the confidence to set off on adventures many would not normally contemplate.
Ultimately, the best guidebook is your own instincts and desires. Each traveler should have a style and set of interests uniquely personal. By all means, travel with a guidebook, but remember it is only a guide, not a Holy Scripture of Travelling. Use it as a reference, but be open to the flow and rhythm of the road. There is a synchronicity of events that overtakes the traveler. Be open to it. Travel is far more than a pilgrimage to venerable ruins or rare sights. It is a process of experiencing and understanding other cultures while discovering the basic humanity shared by all on this incredible planet. Travel, ideally, broadens vision, shrinks human differences, and involves not only tramping through the physical world, but also the exploration of the vast expanses of the mind within.
There are several guidebooks for the physical exploration of India, and more seem to arrive regularly. No guide book is comprehensive, and all will be out of date to one degree or another. Use prices that they quote only as a guideline. (Since prices are often out of date, even a second hand guidebook will serve.) All guidebooks are big. India is a huge country and they try and get everything in.
Here is a list of a few of them, with links to their web sites.
The Lonely Planet
The granddaddy of guide books.
With photos and maps. Being the oldest and most commonly carried, it takes a lot of flack from those that sneer at guidebooks. Critics feel it 'directs' the tourist too much. Good web site. You can post travel related questions in their "Thorntree" section.
Rough Guide to India
Produced by the popular Rough Guide Series, it's a good, comprehensive guide with good background detail on locations. No photos, but sketches and maps. Good organization.
Government of India Tourist Offices
No guidebook, but offices in most major centers offer brochures, tours, and local information. Unfortunately, the web site is stodgy and superficial. India, as a destination, deserves much more.