First of all, you will have to download 4K YouTube to MP3 app from the internet. After downloading the file, locate it and run it on your computer to install it. You can install it in Windows, Mac OS X or Linux.
Then, find the audio book at YouTube which you want to download. Just enter the name of your book in the search bar which is provided at the top of the webpage.
After opening the webpage of your audio book, copy the URL and paste it in the 4K YouTube to MP3 app.
Downloading will take few minutes after which the software will convert the downloaded file into MP3.
Now, locate your MP3 file and open it to listen to your audio book.
You can also get your audio book from the official websites of the publishers. But, you will have to pay some large amount of money to buy it.