If you are in the middle of reading an eBook, be sure to sign into the same Adobe ID account! If you create a new account, the eBook you are reading will no longer be authorized to you and you will not be able to continue reading the book.
Note the "Forgot password?" link to have your password sent to your email account.
Android users will not need to sign back into their Adobe accounts. Apple users who only listen to MP3 audiobooks on the app will not need to sign in with an Adobe ID.