This is the 258th most frequent Finnish word.
jonkin
"Jonkin" means "of something" in English.
In this sentence, 'jonkin' is used as an indefinite pronoun, denoting an unspecified book.
Tarvitsen jonkin kirjan tästä aiheesta.
I need some book about this topic.
In this context, 'jonkin' is possessive, implying belonging to an unspecified but notable entity.
Tämä on jonkin arvokasta.
This belongs to someone of importance.
Here, 'jonkin' specifies an unspecified color, showing a property of the object in question.