Tip: if you have multiple #Audible accounts, you can switch between them in the iOS app and play all downloaded titles regardless of which account it's currently signed into.

For example if your main account is with the US marketplace but you've purchased some titles on the UK site, you can switch to the UK account, download those titles, and then switch back to your main US login. The UK titles will still be playable under the Library tab alongside your US ones.