If you are ever in Fiskars on a Saturday there is a Red Cross flea market on the first Saturday in the month, in a huge red barn at the far end of the village (past the main carpark). From April to October only and it only opens for a couple of hours:
http://www.fiskarsvillage.fi Everything is REALLY cheap (it has all been donated), so books are usually 0,50c each. It's very crowded and very dusty but there are some great bargains (and tons of rubbish).
The FIDA stores are quite good for books and CDs (there is one in Niittykumpu in Espoo and one near the market in Hakaniemi and probably several others dotted around).
Then there is Akseli in Mankaa (Espoo), on top of Citymarket, which is more expensive but generally the stuff is in good condition.