Hi,
Thanks for contacting us here.
Are you using this theme https://themespride.com/demo/stationary-bookstore-pro/? And you are referring sign button. right? If we are correct then you must be using woo commerce to sell the books. In woo commerce there is a page named “My Account”. It contains the Account information of the customer. You need to add its link to that sign in button.
Please try and let us know if you have any questions.
Thanks