itunes audiobooks gone from library

Hi,

I´ve downloaded an audiobook from the ibooks store some weeks ago and even started to listen to it.

I had puschest it from my ipad mini and was surprised that I couldn´t find it on my iphone 6 or my imac. iCloud sync is on though. All other items had been synced before so I do not really understand this. Today I wanted to resume listening but it´s gone from my ipad too..... can´t figure out how that could have happened. When I try to purchase it again in the store (it´s quite expensive....) it says that I have already purchased the Item before.... I Don´t really want to buy it again, it was just to make sure that I had really bought that audiobook. Can you help me please?? I have no clue whatsoever.......

Thanks

Sams