At first glance, the Read-O app doesn't look much different from other digital bookstores: book titles are shown against a dark background, recommendations and a search bar are on the start page.

What's special is underneath: seven sliders, which you can use to determine whether the book selection should be more tragic, disturbing, harmonious or even erotic.

For example, if you combine humorous with disturbing, you will receive the thriller "The Program" by V. S. Gerling or Timur Vermes' satirical novel "Er ist wieder da" as suggestions.

Behind it is the Frankfurt start-up Read-O.

The developers appear on Monday night's show The Lion's Den, which is regularly watched by more than two million viewers.

They offer the five prominent investors fifteen percent of their company for 600,000 euros.

This corresponds to an enterprise value of four million euros.

Emotions as a new category

"We don't recommend books according to standard, well-known criteria, but have invented a whole new category: emotions," says Ben Kohz, one of the five founders.

The books available through the app have been classified into fourteen sentiments based on reviews.

The reviews come from publicly accessible platforms or from buyers who have already used the app.

Genre and sales figures play no role in the app's recommendations.

The app aims to solve a problem Jonathan Mondorf encountered four years ago.

The business administration student noticed that it was extremely difficult to get his father's books in front of the right target group.

The purchase recommendations on the Internet and in bookstores are usually based on the sales figures.

Unknown writers could hardly reach their target readership.

Mondorf sought a team of founders, each with different skills.

The five men, between their mid-20s and 30s, created the app over two years.

They were partly financed by the state Exist start-up grant.

Funding through affiliate links

They now also have private investors.

The app now contains around 500,000 books.

Those who find it difficult to express their taste in books in the fourteen given emotions state their favorite books and receive recommendations as a result.

Before the "Lion's Den" recording, the app had 70,000 users, according to the founders.

It is financed through affiliate links: When users order books in the app, Read-O receives a small commission from the retailers.

Users do not have to pay anything initially, but recommendations can only be saved in the long term with a paid subscription.

The app is an addition to the book market, says co-founder Kohz, because it mediates directly between readers and books.

Around 70,000 new books are published in Germany every year.

Although sales in the industry are stagnating at 9.3 billion euros, the share of internet sales is increasing.

Every fourth euro is earned through online trading.

The Read-O app competes with Amazon, Thalia and Google, for example, who have been displaying book recommendations in their shop apps for years.

With around 10,000 downloads and 283 ratings in the Android Play Store, the Read-O is still far behind the market leaders.

Nevertheless, the start-up would like to be “an integral part of the book industry” in a few years.