Step 1. Check if You Have Downloaded iTunes Audiobooks to PC
Open iTunes and signed into iTunes with the same Apple ID that you use for purchasing the iTunes M4B audiobooks.
Go to "Audiobooks" and see if you have downloaded the purchased audiobooks from iTunes Store. If you can't find your purchased audiobooks in the "Library" section, then you need to download the audiobooks manually.
Step 2. Select iTunes Audiobooks in TuneFab Apple Music Converter
When all your purchased iTunes audiobooks are downloaded in iTunes, you can open TuneFab Apple Music Converter and choose the audiobooks you like for conversion.
Go to "Audiobooks" section, then all the audiobooks stored in iTunes will be displayed in the middle of the main interface. You can sort out the audiobooks by click at "Type". You can choose the audiobook simply by ticking at the checkbox.
If you cannot find the audiobooks in iTunes, click at "Refresh" button to update the library.