Which is correct: "I burnt the book", or "I burned the book?"?

"Burnt" is only an adjective, not a verb, in American English. I burned the book. Now the book is burnt. (Equally correct: Now the book is burned.)

In British English, "burnt" is both an adjective and a verb.

Either one. "Burnt" is used more in the UK than in the US, where it is sometimes considered old-fashioned.

It depends which dialect of English you re speaking.

both

Either, however.

~I burnt the book~ scans better.

I burned

I burned the book. The book was burnt.

The second one. The first would be used by saying "I burned the book. It's burnt."

I burned the book. The first one is too wordy.