Our token system was initiated because the email address you used to try to make the purchase has been used previously to make a Leanpub purchase.
If you check your email, you should see an email from Leanpub containing a short string of characters. If you copy this "token", then you can paste it into the screen where you are being told about the token. Then, you can log in to Leanpub and complete your purchase.
Alternatively, you can ignore the token and sign in to Leanpub. You can do that here: https://leanpub.com/login.
Here's how this works: we automatically create an account for you when you first purchase a Leanpub book, and we create a library for you at https://leanpub.com/user_dashboard/library. This is done so that you can always access the download links for latest version of all the books you buy, all from one place.
If you've never set a password for your account, you can do this using the "Forgot Password?" link on our login page: https://leanpub.com/login. Once you are logged in, you'll be able to go to your Leanpub Library.
Once you have logged in, you will not encounter the token system. Then, you can make a purchase as you normally would.