If you originally created an account using an email address and password, and later log in using Facebook, Google, or Twitter, then Perusall will create a second account for you, and you won't have access to the courses or purchased books from your previous account. As a result, make sure you use the same method of signing in consistently -- e.g., if you originally signed in through Google, continue signing in through Google; if you originally created an account with an email address and password, continue signing in using that email address and password.
If you forget your password for an account you created using an email address and password, use the Forgot Password link on the login page to have a reset password link emailed to you.
For security reasons, Perusall is very case-sensitive with information. If you attempt to log in with information and are prompted to create a new account, check the typing case of your email -- e.g., email@example.com vs Johndoe@institution.edu.