1. It's fascinating.
2. It's intriguing.
3. I couldn't tear myself away.
4. I couldn't put it down. (this phrase is used for en extremely interesting book)
5. I was so into it, I lost track of time.
6. It does nothing for me.
7. I was bored to tears.
8. I was bored to death.
9. I was dying of boredom.
10. It's about as exciting as watching paint dry. (= it's very boring)