i have read the wikipedia rules a few times and as far as i can find it says all reliable sources are by definition both published and accessible to at least some people sources that are not published e g something someone said to you personally or not accessible e g the only remaining copy of the book is locked in a vault with no one allowed to read it are never acceptable as sources on wikipedia in my interpretation i can refer to jahre handel und kultur im ostseegebiet gotland perle der ostsee isbn as far as i can see this book qualifies if you accept this we can go further if you search isbn on google you will get results openfuture does not accept this are you reading the rules in the same way