I think its not your older books, but all books that do not fit the new 'death + 70 years' copyright rule used by Mobileread.
For example, one of the offline books is 'Williams, Valentine: The Yellow Streak'. According to wikipedia V.Williams died 1946. Following the rule, the book will be available again on Jan 1st. 2017.
I have no idea if there is any way for you to get the book, but anyone else can probably just wait.
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