There are two possible solution for your situation:
Without In-App Purchase: You can create an app which can have a eBook (in your case, dummy eBook with chapter 1 content) bundled with the App. By doing this anyone who can download will have access to the dummy eBook as well.
For downloading of additional-contents user can do login and access the eBooks available with their account. Kindly note as you are not using In-App purchase there shall be no links for user registration, buy inside your app. All these you can handle at your web-site. Your app can check for valid user credential using a web-service call, and user can see the contents available for download.
Advantages: 1) You shall not be paying 30 % to Apple as No-purchases are done using your app. 2) This is total compliance with Apple guidelines, this link and part 11 specially will be of interest. Disadvantage: Users cannot directly buy/register via app and you need to maintain additional services for user login validation/content mapping/download content etc.
Using in-App Purchase: You can use Apple provided in-App purchase for selling content/ registration of users etc. Please note Apple shall charge 30 % on your selling price.
Advantages: Users can register and buy directly using your app. In-App purchase guidelines will help more in this. Disadvantage: Apple will charge 30% on Selling price.