You probably can't find book store around National Park, but I think you can find it in sovenir shop. Hotels around the national park are very adapted to bird birdwatcher, they provide updated bird information. I remember there is a small shop (at least one) next to Hotel Sunbird selling sovenir as well as small bird book.
In Delhi, you can find bookstore at Connaught Place (Central Park). However the book stores are dim and chaotic, books are always covered by dirt.
Indian know very well how to earn tourist money. In Ranthambhore NP, I bough a photo guide through a road side toutor and the price is cheaper than in book store after bargaining. Never tried to bargain on fixed priced book! Very amazing.
Gary
