I would like to know if there is any book/magazine/written reference with places in Finland where I can advertise a service for a company.
The company deals with web design, so the target is everyone who has a business, therefore it is not a matter of the right target.
There are several lists and registers in the internet, unfortunately many of them only in Finnish. You can make a search with words like "Suomen yritykset" and get several registers and search systems.
Here is a link for a good regional search, only in Finnish:
The home page of the Finnish Business Information System: