When I was in Finland recently, I bought a couple of books. First I found "Totta" in a cheap paperback edition, then I found "True". Both of these I found at Akateeminen Kirjakauppa. So I can read the book and improve my Finnish vocabulary at the same time.
Because of increased immigration nowadays, several more useful books have been published to what was available when I was last in Helsinki, many years ago. One useful book for foreigners that hope to actually understand what people are saying is called Oikeeta suomee - Dictionary of spoken Finnish, which has a useful introduction explaining various regular contractions in the spoken, as opposed to written, language. As in any language, there is some difference between the written and spoken versions, the differences often involving shortening things. Written Finnish does strike me as exceptionally long. Sometimes there are colloquial or slang expressions which are completely different to the standard language. Helsinki slang, for instance, has a lot of Swedish words in it.