This deletes your account, your movies and books, their dates, ratings and comments, your friends and recommendations, and the pictures you uploaded, from the server and so from every device that syncs. It can't be undone.
The quickest way is in the app: open Settings and choose Delete Account. You can also do it here with your email and password:
Signed up with Apple or Google and never set a password? Delete the account in the app, or set a password there first (Settings → Set a Password).
Questions about your data: vilbrooksstudio@gmail.com.