Need tips on adding an auth layer to an existing webapp

[KenA]

Hello. I have a working webapp that I'd like to add an authorization layer, so only authorized users can access it. It's a simple scheme since there will not be different Roles and/or certain folders/paths to be distinguished as allowed or not. Once the user is granted access, he/she will have access to the entire app.
This app is actually a support app for an ebook i'll be selling, so the idea is to grant access only to users who bought the ebook. For example, suppose the user buys the ebook at Amazon. As far as I know there's no way amazon would re-direct the user to the app's url, neither send a token with info so the app would use to register the user. What would be the best way to accomplish it?

 

[Jacqualine]

hmm, it is difficult .. but I think if you register your ebook to some listings or product introduction websites from there they can give a link to your app URL so that users can register.