Should audio books that are purchased appear in cloud?

That's a good question and until the service is fully functional we won't know. I have read that iTunes Match will not add your apps, books, movies, TV shows, ringtones, or audiobooks to your iCloud library. Those will continue to be synced to your iOS device with iTunes as they are today. iTunes Match will only "store" your music library. I've also read that iTunes "LPs" and iTunes "Extras" won't be part of the Match service either.

Apple has only discussed actually "storing" music, books (ibooks), and photos (plus syncing calendars, etc.) as iCloud features. They say...

iCloud stores your music, photos, apps, calendars, documents, and more. And wirelessly pushes them to all your devices — automatically. It’s the easiest way to manage your content. Because now you don’t have to.

So there's a bit of a conflict and what "more" means only time will tell.

For those of us that have ripped audiobook CD's and/or buy audiobooks via iTunes or places like audible.com, having them available via iCloud would be great. However it looks like we'll have to wait and see as they say.