Also, you can say it after you hear someone else saying that he or she didn't like the book.

When, after reading the book which you thought was quite boring, you found out that it was interesting

It's a subtle difference. Both of these would be responses to the statement "Tony's book was boring" or something similar. But you read the book, you liked it, and you thought it was interesting. You could then say "Actually, I thought Tony's book was interesting." But imagine that before you read it, you didn't think you'd like it. Either you've read other books like it and you hate them, or it's about a subject you're not interested in, or it's science fiction and you don't like science fiction. For whatever reason, you did not think you'd like the book. Then you would say "Actually, I thought Tony’s book was surprisingly interesting." The "surprisingly" means you did not expect it to be interesting, and it was. That interest was a surprise!

Tell me I don't understand was Tony’s book interesting or did it suck? I can't understand.

haha. "would have thought" = hypothesising about the past. "I thought " is also correct.