This isn't what you wanted, but I've been using the Podcast Republic app as a way to organize and listen to my (usually free) audiobooks. It's android only, so

This isn't what you wanted, but I've been using the Podcast Republic app as a way to organize and listen to my (usually free) audiobooks. It's android only, so only on-device and not on PC. Its audiobook support is a bit obscure, it relies on creating what they call a "virtual podcast" based on the filesystem. But once you find it it's pretty simple and fits into their UI, and you can tag / catalog your audiobooks as much as you want. I also like its listening UI. It remembers where I last stopped and starts playing a configurable number of seconds before, remembers playback speed per book/podcast, remembers audio settings per book/podcast, has a car mode. I like it much better than Audible's UI. Like I said its organizing UI is also nice, although I use it less, I don't collect audiobooks on my mobile, due to the limited internal capacity.