Both are correct. "Who wrote" is simple past tense. "Who wrote this book?", "I love this song, who wrote it?" "Who did write" would be used in response to a negation. "Well if Shakespeare didn't write this sonnet then who did write it?

Both are correct. Someone might use "did" for emphasis. "He was not the author." "Well, who did write that book?"

Both are right, the first one is a question, the second is an emphasis.

Who was it written by? Another way to ask about that. :D

Both are right. :))