@smartie try it on any books that you're interested in.
Of course, it will apply to all books on the India site for example, and other sites where publisher restrictions don't hold sway.
However, 20% off is not a good code unless you're in a hurry and have used all the codes on offer via the Discounts thread. Also a new comp is due in 1 week or so. My suggestion would be to hang in unless you need it right now.