[ApK]

I have not had much success with the dbPowerAmp or older Goldwave methods.
I've used Soundtaxi in the past, and if you have a decent multicore machine, and the book is in multiple parts, it should be able to do the parts in parallel, saving quite a lot of time.

For me personally, I've given up trying to convert Audible books. The books I get from them are always highly discounted, so I have little invested in them.
90% of what I get from Audible I will never listen to a second time, so for me, most of the time, Audible might as well be a pay-as-you-go borrowing service.
What few audio-books I want in my permanent collection and really feel the need to protect future access to, I can get from other sources.
And the devices I play them on all have Audible apps now, so it's just too convenient to download directly and leave them alone.