My private view: I don't support this kind of ideas. Just either let the damn people read your book, or make them pay for it, or go away. Don't gamify everything, people. It's annoying. Especially if you want to "reward" someone for visiting your websites that they're probably not excited to visit.
Okay, that being said, if you want to give tokens, you'd need people to have their accounts. Of course, you could in theory do it without user account - storing tokens by IP or cookies - by it's not a good idea. IPs are not necessarily static and can be shared by hundreds of people. And cookies can vanish as soon as you clear them. Cookies are also not portable, they are stored per device. And we usually use more than one device today.
You could use Firebase to complete this on the backend, but I'm not sure how experienced you are.