This is the 197th most frequent Finnish word.
siinä
"Siinä" means "there" or "in it" in English, depending on context.
The word 'siinä' is used here as a demonstrative pronoun referring to a specific object (the book).
Siinä kirjassa on paljon mielenkiintoisia kuvia.
There are many interesting pictures in that book.
The word 'siinä' is used here as a pronoun emphasizing the mentioned issue.
Siinä se ongelma olikin, että aikataulu oli liian tiukka.
The problem was indeed that the schedule was too tight.
The word 'siinä' is employed here to specify which house is being described.