This question has been simmering for quite a while, but I figured I'd answer.
I would recommend Bound. I've used it quite a bit, though I don't have much experience with other audiobook apps. It can download audiobooks from cloud storage services (google drive, dropbox, etc..). I use it by dumping all my audiobooks in dropbox and just download a few books at a time and remove them when I'm done. Being able to download the books is way better than using wifi upload or a cable.
Bound can batch download all audio files in a folder in the background. My books, for the most part, are sorted in separate folders and each chapter is in its own audio file.
It doesn't have something like folders to organize book series, but as I only download a few books at a time, it doesn't bother me. Also, Audible's mobile app is the same in that regard so I'm used to it.
I did recently learn about CloudBeats, which seems to support streaming from cloud services and I guess it did work for one chapter at least... I'm not too sure if I used it right... Seemed a bit difficult to keep track of what was last played and even though I made sure that "resume where you left off" was turned on in the settings, my stream didn't continue where I left off. Has potential, but I think I'll stick to Bound...