Nope, as you've discovered Apple has removed support for audiobook playlists in 8.4. Yet another slow erosion of the more useful "power user" features of iTunes.
I used to have several audiobooks on the go, depending on mood, along with hours of radio drama and comedy which I imported into iTunes and defined as audiobooks to keep them out of my music collection. All of these were curated using some quite complex Smart Playlists, then the unplayed stuff (plays=0) was synced to the iPhone.
All of this is now broken. If I want to listen to the next unplayed audiobooks or radio shows I have to either pre-select them in iTunes before syncing, or sync everything then select them manually in the iBooks app. It's a massive step backwards. It also appears that I can't use Siri hands-free in the car to select audiobooks. I could do this when they were in the Music app, by requesting their playlists. So thanks for that, Apple.
I've partially worked around the radio shows problem by defining them as podcasts rather than audiobooks. I can select them in the Podcasts app, and also via Siri. But when I sync them it also copies all of the "real" podcasts from my phone back to iTunes. Less than ideal. And the "album art" doesn't display either. And it's no help for genuine audiobooks.
My brief dalliance with Apple Music was disastrous too -- all of my album art got screwed up and for every track I'd given a star rating iTunes saw fit to give the same rating to the whole album, so my "favourites" (four- and five-star) playlist jumped from 130 tracks to over 2000 because every album got added to it. Took nearly two hours to sort that out.
I get the distinct impression that Apple doesn't want people curating their own media collections any more. They want everyone to sign up for iTunes Match / Apple Music and just let Apple do everything. And while I know this works wonderfully for some people, especially those without a PC who don't use iTunes, it's just frustrating for those of us used to doing things our own way.
I find it difficult to believe that none of the beta testers mentioned the Smart Playlists / audiobook / iBooks problem during the test phase, so can only assume that Apple simply ignored those people and ploughed these changes through anyway. I now fear that we are one, maybe two new iTunes releases away from losing Smart Playlists altogether. They'll just remove that feature. At which point iTunes will become practically useless to me. 😟