"I need my book."
Translation:Jag behöver min bok.
You're on the right track, but sin is used for third person, min for first person singular. So sin means "his/her/its own", not "my own".
Here's a full list of reflexive possessive pronouns (common gender):
- jag - min
- du - din
- han/hon/den/det - sin
- vi - vår
- ni - er
- de - sin
Oh, thanks, I was comparing it to my language (moj/svoj (my/my own)) and since "svoj" starts with an "s" the same as "sin" I thought min and sin are interchangeable. My bad :)