I will be going to Copenhagen, Stockholm, Helsinki, Tallinn. Any suggestions on city maps/ guidebooks that would be of help to me for these cities? I am planning on just doing a walking tour of these cities. Don't have time for a full day in Copenhagen as the ship is only there for half day so I think watching the changing of the guards in Copenhagen is out as the ship leaves at noon. Thanks.

Let me address Helsinki specifically - it's a beautiful, clean city and the Finns are a delightful people BUT if you can manage it and if its offered, take the flight to the Arctic Circle to visit and meet the Sammi people. They raise reindeer (the way we raise cattle) and are a beautiful, fascinating people. It's an experience you'll never forget!

You might want to also check out the hop on hop off buses. Most of these cities have a tourist office close to the ports you can get information.