Transfer audiobooks to iBooks to be able to use my Apple Watch again

So for two years, I’ve been using the Audible app to listen to books from my watch, so I don’t have to have my phone with me all the time. Now my Audible app doesn’t work right so I wanna be able to download my audible books to iBooks so I can listen to it on my watch without having had my phone with me. I’m not sure how to do that. I’d like to be able to do it just on my phone without having to get off the computer so then it’ll transfer straight to my watch.

[Re-Titled by Moderator]