Apple has long been offering a book app for the iPhone and iPad that downloads and reads electronic books and audiobooks.  WatchOS 6 lets you listen to audiobooks on an Apple Watch. Find and buy audiobooks from your iPhone or iPad that you can download to your device. Then you can add audiobooks to your watch to listen to wherever you do not want to carry them around.
First, it explains how to download and use audiobooks on your iPhone. It then explains how to listen to them on your watch.