We have to advice you that the following not an easy task: finding English books in Santiago. Of course, you can always use E-Books, but we understand that from time to time, it is nice to feel the cover and the paper of a book. You can also have other strategies such as bring books from your country (which is heavy and requires space in your luggages, always difficult to have) or have the book delivered via a courier.
Searching we found some places where you can buy books here in Santiago. You should know that books are a little more expensive that in other countries.
1) Librería Inglesa: You can find stores in different places like Vitacura or Providencia. They have different kind of books in English. At this time of the year, they have more English teaching books (schools classes are starting in March here in Chile), but they receive more variety of titles from April/May.
2) Feria Permanente del Libro Usado de Providencia: it is situated in Providencia 1114. You can find second hand books in English. It can be a good option to find books at a cheaper price.
3) In the books stores Que Leo, Antártica and Feria Chilena del Libro, you can also find an English book section, but probably is no so big.
If you have another data, please share with Inside Santiago community, we will appreciate!