During his lifetime, Larni was one of Finland's most internationally known writers in the Soviet Union and other Eastern Bloc countries thanks to his book Neljäs nikama (The Fourth Vertebra).

While attending public school, he was a shepherd at his mother's house in Hauho during the summers; these experiences later contributed to the book Hyvien ihmisten kylä (The Village of Good People).

His next book, Kuilu, published in 1937, caused an uproar because it dealt with sensitive topics such as the Finnish civil war (from the point of view of the Reds) and homosexuality.

In 1957, based on his experiences in the United States, Larni wrote the book Neljäs nikama eli Veijari vastoin tahtoaan (The Fourth Vertebra, or a Scamp Despite Himself), a satire on American society with its quick marriages, commercialism, and ignorance of the rest of the world.

The book's title comes from the fact that one of its characters, the chiropractor Isaac Rivers, believes that all back diseases are caused by the fourth vertebra of the spine.