How to transfer audio books from my PC to itunes?

I downloaded an audio book to my computer and I need to put the file into iTunes so i can put it on my iPhone

If you have audio or video files on your computer, you can import them into iTunes so that they appear in your iTunes library.

To add media from MusicMatch Jukebox or Audible Manager to your iPod, iPhone, or iPad, you need to first import it into iTunes.
Here are ways to add items from your computer to your iTunes library:
1. Drag a file or folder from the desktop to the iTunes window.
2. If you add a folder, all the files it contains are added to your library.
3.In iTunes, choose File > "Add to Library," locate a file or folder, and then click Choose.

Yeah, the answer upstair is right and it is how we sync files to iTunes, but you need to make sure the format of your audio book is available for your iPhone, cause ios devices only can support a few formats like mp3, mp4
here is the tool that can convert audio format (if you need)
http://www.bestaudioconverter.com/

You can add the audiobook to iTunes by simply dragging it to the iTunes interface. But you should firstly click the Books tab in the left bar.