Adding book without author?

[Tuula]

I've some books which have no author, e.g. one from year 1861. https://finna.fi/Record/fennica.62039... . So question is how to mark the author in this case, as author is mandatory field?

[Scott]

Anonymous, I guess.

[rivka, Former Moderator]

The choices are either Anonymous or Unknown. Which is best in a given case varies.