This is how he weeds out the ordinary customers.
Go to the bookstore with an old book and a golden pin in your left hand. Prick your right ring finger with the pin and write on the front window "I have a book" with your blood.
It doesn't matter how small the prick is; enough blood will come out for the message. The owner will open the door for you. He is a small, wiry man with a hunchback and one glaring glass eye. He will favor you with a crooked grin and tell you that in exchange for the book you hold, you may take one book away with you.
Should you peruse the shop, you will find a book. This book will change your life to what you want it to be. Be warned: Not all the books are as helpful as they appear on the surface. None of them reveal how to change your life by non-ritual means.