Website
For the website, you can hover above your profile picture and select "My Reading Lists." From there, select the respective reading list, select the story, and hover above "Add to Reading List." You will see a checkmark on the list, you can just click on it and it will then remove the book from your library.
App
For the mobile app, once you tap the "Library" button on the bottom menu bar, you will see your books under "Current Reads." Click on an individual story, and then you'll see it marked as "Saved" in the bottom menu bar. You can click on that bookmark icon, and then just uncheck the reading list and it will be removed from your library.