To implement this feature on your website, you have to download open-source code from https://github.com/satorumurmur/bibi

On the Github, webpage click the Code button and select Download zip. Then download the bibi-master.zip file to your Downloads directory or a directory of your choice.

Unzip the bibi-master.zip file, and you will see several files and folders. Open the '_src' folder, and you will see three folders: bibi,
bibi-bookshelf and bibi-demo.

Create an ebook directory in your website root directory. Upload the bibi and bibi-bookshelf folders to your ebook directory.

Upload your reader magnet in epub format to the bibi-bookshelf directory.

Now to let a visitor flip your ebook pages, compose the following link:
<a href="yourwebsite.com/ebook/bibi/?book=magnet.epub">Reader Magnet<a>

Here is my reader magnet, you can flip its pages and read: http://sergeyskudaev.com/ebook/bibi/?book=magnet.epub

At the end of your reader magnet, you can place a link to your subscription form.

Suppose you are afraid that a visitor will not be willing to subscribe to your email list after reading your reader magnet. You may create a short version of your reader magnet to allow reading on the webpage and a long version to download after subscription.