Questions & Answers about Laitan kirjan siihen.
Laitan is the first person singular form of the verb laittaa.
It means “I put / I will put” depending on context.
Finnish does not have a separate future tense, so the present form laitan can mean either I am putting (now) or I will put (later).
The subject “I” is included in the ending -n, so you do not need to say minä unless you want to emphasize it (Minä laitan kirjan siihen = I (and not someone else) will put the book there).
Kirjan is the object form (often called the accusative/genitive form) of kirja (book).
When the object is a whole, complete thing and the action is seen as complete, Finnish often uses this case: kirja → kirjan.
So laitan kirjan = I put the (whole) book.
If you said laitan kirjaa, that would suggest an ongoing / incomplete action or putting some of a book in some context, which usually doesn’t make sense here.