Currently, in iOS 8.3, a users' audiobook library is divorced from their ebook library, forcing users to use two different apps to access their entire digital book library. The new update consolidates all books in Apple's iBooks app and also fixes another longstanding issue with audiobooks on iOS. Since iOS 7, audiobook listeners have been unable to view chapter titles or even select them. However, pazimi points out that while chapters are now selectable in iOS 8.4, they do not list the actual chapter titles, as they're listed as "track numbers".
Finally, Apple has added a new fast forward and fast rewind feature, allowing users to easily scrub through an audiobook chapter. Users simply have to drag the book cover art to the left or right and hold to jump back or move forward further within the audiobook.
Update: iOS 8.4 also adds a standalone Audiobooks app to the CarPlay interface.