Audiobooks occupy a weird backwater on iOS. Long tucked away in Apple's Music app behind a 'More' button, audiobooks were kicked out of Music with iOS 8.4 and now live alongside eBooks in iBooks. Audiobooks are also one of the most restrictive types of media you can purchase on iTunes. Once downloaded to a device, an audiobook cannot be re-downloaded. Download an audiobook to an iOS device that isn't backed up and if your iPhone or iPad dies, your audiobook dies with it. iTunes audiobooks are, to borrow a Steve Jobs' complaint about Blu-ray disks, "a big bag of hurt."
iTunes is not, however, the only game in town. Services like Audible let you re-download books, and there are plenty DRM-free audiobooks available. But Audible is its own proprietary system and even DRM-free audiobooks cannot sync to iBooks on an iOS device without using a Mac and a cable. Pain points like these are what create opportunities for third party developers like Tim Bueno who has taken the pain out of getting DRM-free audiobooks onto your iPhone with a promising, although imperfect, Dropbox-connected audiobook player called Bound.