I have to join the conversation, for "really" is my favorite word. Really!
I do NOT say that my opinion is correct. But it satisfies me.
Tom: It is a good book.
Mona: How good?
Tom: Really good. (The adverb modifies "good.")
*****
Tom: It is a good book.
Mona: I don't believe you. Are you joking?
Tom: No, I am not. Really, it is a good book. / No, I am not. It is [,]really [,] a good book. / No, I am not. It is a good book, really! (The adverb modifies the whole sentence. It means something like "Believe me!")