Questions & Answers about Minä luen kirjaa metsässä.
Kirjaa is the partitive case of kirja (“book”).
In this sentence, kirjaa is used because the action of reading is ongoing / incomplete or not seen as a finished whole. You’re just “reading (some of) a book”, not necessarily finishing it.
- Minä luen kirjaa. = I am (in the middle of) reading a book.
- Minä luen kirjan. = I (will) read the book (from start to finish, complete it).
So kirjaa highlights that the reading is a process and the object is not viewed as a completed whole in this context.