I also use Audiobook Builder to convert mp3 spoken word content into .m4b files. To listen to them I’ve settled on Book Mower, primarily for the (rather opinionated) UI. I find it much easier to navigate when I’m on the move, listening while doing chores, or listening at bedtime than the other apps I’ve tried. It has GIANT tap targets which are much easier to navigate if you’re outside on the go or have soapy hands or whatever. It does some “automatic” bookmarking that’s more fussy than useful, but does have a decent tool for marking a specific place you might want to go back to.
You can import files directly from your iTunes / Books library or from another device on your local network via a web browser. I like the interface so much that I’ll jump through a couple of hoops to get content in there from sources that have their own players (e.g. Audible or Libro.fm).
One caution: I’m not convinced that the app gets much active development and there are occasional playback glitches on bluetooth.
I tried BookPlayer, but haven’t been able to actually load any content into it. AirDrop doesn’t work as advertised and it apparently doesn’t pull in files directly from iTunes / Books.