The book I wrote my essay on is very boring.
The book you had been looking for was behind the wardrobe.
This is the woman about whom I’ve already told you.
This is the book about which I told you.
The doctor who wrote this book got a Nobel prize.
Dr. Smit, who wrote this book, got a Nobel prize.
My sister who lives in Canada got married.
My sister, who lives in Canada, got married.