Great question - so, there are a myriad of ways you can look to monetize apps, but there are really three directly available ways, and a few indirect.

Direct revenue generation for your app could include paid (i.e. download cost), in-app purchases (which can include a recurring subscription) and advertising. Indirect revenue generation can include anything from reference links, syndicated/affiliated content placement, sponsorship and lead generation or referrals for purchasing goods and services.

Regarding in-app purchases, they are actually a strong and growing segment of revenue for app makers and app-preneurs (around 48% of all app revenue is generated through in-app purchases, and is targeted to cross the $30B/year mark before 2020. Some entrepreneurs (such as authors for example) will look at mobile apps as a supplemental feature for readers who buy their books, or as a means to generate book and new content sales as they are released (mobile commerce enabled shopping with in-app payment processing etc.)

What is of primary importance with an app like yours is to organize the content in a way that is very intuitive for users to access, but also provides some level of customization or uniqueness for readers. So in creating a viable business model you should start with making sure your app/features/tools are viable as a product for your users and buyers. Making money with apps isn't all that different of a process of thinking than standard ecommerce (and even brick & mortar commerce in certain cases) - it's about streamlining and eliminating bottlenecks,mapping out/following a clearly laid out conversion funnel to move your users through the stages of sign-up, reviewing and interacting with products, purchasing and consuming products and then repeated the process. The viability of the model can largely depend on making sure the product you are offering is calibrated correctly for your intended audience and is guiding people with smart UI/UX through your app towards your ultimate purchase outcome/conversion points.

I've helped dozens of entrepreneurs over the years develop and execute both a business plan and product plan for their new mobile apps, so if you're looking for more direct/specific insights feel free to reach out!