I mostly buy second books to read and collect. The new books I read are rarely bought and mostly borrowed. This post does not claim to be a complete guide to book shopping in Delhi but merely about the places I am familiar with.
The second book shops that I know in Delhi are:
North Delhi: Delhi University, North Campus, head to Kamla Nagar Market and find the roundabout/gol chakkar. From the roundabout, take the lane heading towards Prem Studio, pass by the studio and you hit a smaller roundabout. At the left corner there are two book shops, one of them, respectably called (by me, of course) uncle-ji stocks a good collection. Uncle-ji appears quiet innocent and hearty; just do not test his staying power when it comes to bargaining. He has ripped me off bad on some of the books I was craving for. I was negotiating to buy all the Granta's, uncle-ji could procure for me over time but his prices were outrageous so our deal fell through. This shop has a great collection of coffee table books too. The other shop right next to this stocks a lot of pulp fiction, popular authors and interestingly from what I remember a lot of the comics we read as children, from Amar Chitra Katha's to Chacha Choudhary's.
Central Delhi: This weirdly perched book shop on the side of a wall is right next to Plaza theatre. The collection is interesting. Sometimes fatigue inducing as there are just too many books piled one on top of the other. Browsing by one’s self can be a hazard especially if you try to pull out a title on your own, expect the whole pile avalanching down on you. I can bet you will be run over by a car or an autorickshaw that passes within inches of your butt while you admire the titles. This shop is a favorite with the back pack crowd that stays at Pahar Ganj and hangs out in Connaught Place, so a lot of the books reflect that wide eyed occidental obsession with India. The owner is quiet aware of what he has and is a master haggler, impossible to get a good bargain with him.
South Delhi: PVR Saket is the place I know well and consider the best for a casual low energy browse. The second shops are literally a dime a dozen here. The ratio of good books to area covered is also the most efficient here (comparison is Daryaganj). I have found very interesting books here, from Philip Roth, Paul Auster to Prem Chand and Readers Digest Adaptations. The key is to carefully browse the Special priced ones which they sell for Rs 50.
Vasant Lok Complex or Priya: There is one shop which has a decent stock. I include him in this because he is the closest to me and so it is my regular haunt. I have bought all my Graham Greene’s, JD Salinger and even Henry David Thoreau's Walden from here. He has a sizeable number of classics and a lot of the popular authors. The interesting thing is that he also has a collection of those a little larger than pocket sized commando series comics and photo romances. The shop is located on the main street, on your right if you approach Priya from Nelson Mandela Road just about ten yards before you hit the gas station.
For new books, I do not have preference of shops.
Fact and Fiction at Vasant Lok Complex is closest to me and satisfies my conceit with regard to books. It is small but extremely well stocked and gives me all the narcissism of a high browed reader. I have run into many important people here including Soha Ali Khan, mentioned here for her glamour quotient. It is small so when it gets crowded, it is smarter to get out.
Om Book Shop at Priya again is big and expansive. The downside is, it is merely a business for them. They will find you the books you want but that is all. Don’t even try to stimulate a conversation. While in Fact and Fiction, it’s a pleasure to talk to the owners and I have sometimes been accosted by even fellow customers for a discussion. Om allows you to read but if you stick for long, an old man keeps you in his constant vision. The thing to do is ignore him, look through him and continue reading.
Mid Land: In South Extension (left side if you going towards Moolchand), at a weird corner where they sell fruit juices and salads. It is located in the same lane as Bengali Sweets (horrible food and overcrowded) but at the opposite end. You approach it with a flight of stairs leading to a basement and you are rewarded with a shop as big as Om and with at least double the stock. I discovered Mid Lands quiet late but among the book shops I am familiar with, Mid Land easily stands out. They have the entire Mario Vargos Llosa collection and they even have Norman Lewis (not the word power guy), autobiographies and travel books.