Richard, I am having to guess as, not least, I do not use sound on my GPS / motorcycle. But, here goes:
These audio books, are they from something called ‘Audible’ (‘Audible.com’ or ‘Audible.co.uk’) which seems to be some Amazon offshoot? If so, the following may help you, perhaps:
It seems to be possible, which is an encouraging start:
https://support.garmin.com/en-IE/?faq=ipdSu1zPrIAhWUJYAApol6
Garmin also tells you how to get them off again:
https://support.garmin.com/en-ZA/?faq=jz6ngwjtoTAB4EUlsYjmZ9
Let’s start, do you have the Audible software loaded onto your GPS device? This video seems to show what it is you are looking for:
<iframe width="560" height="315" src="https://www.youtube.com/embed/iFVu5n-cYVo" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
Yes? Great then it seems you are starting to cook on gas.
https://www.tuneskit.com/audiobook-tips/play-audible-books-on-garmin-gps.html
Assuming we are talking about ‘Audible’ books it seems to be only possible to do things through a PC, not a Mac. I did though find out, I think, that if you first set up the GPS and ‘Audible’ through a PC, you can do the transfer via a Mac thereafter.
Over to you to see if the above is on the right lines. If it isn’t please tell us what precisely what software for the audio books you are using, their file types, whether you are using a PC or a Mac etc etc etc
Failing all or any of that, try the Garmin 590 forum:
https://www.zumouserforums.co.uk/community/viewforum.php?f=12