Shankar market , Janpath , nai Sarak, Daryaganj and cp stores are some famous spots for getting cheap books around the city
Daryaganj market is the known and best place, even for second hand books. But you can also go to Janpath for cheap novels and can even trade one for another
Daryaganj market but I guess it has been shifted now
Daryaganj and Janpath are the best places to buy books.