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!

I am assuming that your basic question is how to make money by reusing quotations from books, particularly spiritual.

This idea is out there but if your quotations are amenable to memes then you could create a collection of them and offer them as a subscription or sell them on stock photography sites.

There are a number of meditation type apps that are filled with quotations. You could approach them to see if they would like to license your content. It doesn't seem like a huge market but as a developer of one of these apps I can see having this thought "I need to get inspirational quotes to go here and I don't want to build my own database of them...can I subscribe to an API that will give me these?"

It seems to me your quotations could help lead people to buy your books. If you were looking at all quotations in your business then how about an app that helps generate quotations from a book and turn them into ready made ads?

If I were looking for spiritual books and I can see myself browsing a collection of quotations from a book before deciding to buy, as this would help me sample the feeling the book may give me.

Put this all together I can see an inspirational quote service but I don't know if it would drive sales. I think you would do better to find a way to "disintermediate" amazon on spiritual books. A quotation becomes a sort of meme that you share on social media that drives people to this site. So they are to some extent prequalifying themselves since the quotation resonated with them. If they then see a number of quotations that resonate further they may choose to purchase a book.

So that's a lot of spitballing. I hope something in here helps trigger an idea for you to move this forward so you can realize the most from your writings.

Blogging is the right solution for you. Setup a blog on a particular interest/topic that you wish to write upon. Then you should start posting catchy lines on social media that attracts readers to click on the link and read more at your blog. When they visit your blog, they will see the Ads too. Ads will fetch you the revenue that you are looking for.

Let me give you an example:

"The Snowman came to life and started throwing stones towards the group of people. This was an amazing and shocking view for the spectators. Finally the Panda jumped... (read more)"

This is called information appeal, people will click that link out of curiosity.

I am an expert, I can help you step by step in generating a great business and fetching profits. It will become passive income that will be generated 24/7 as your blog will work for you even when you are asleep, even when you go out on a dinner with family. It will be working on and on for you. One visitor will bring other visitor and they all will bring visitors to your blog.

Setup a call if you wish to consult me.

Regards
Shishir