Are you searching for something new to read? Then have a look at Senaatintorin Antikvariaatti in Torikortteli. Here you can get book in Finnish, Swedish but also in English for prices as low as 20 cents.
But also if you want to get rid of some of your old books you should drop by. If they are in good shape they might buy them off of you.
Have a look at the small shop in Sofiankatu and for more information also visit them online.