I bought my iPad Mini back in 2013 to replace my wonky Nook ST. Its primary purpose is to be an eReader, not an all-purpose tablet.
I have resisted the urge to upgrade, and am running older versions of several applications:
iOS 7.1
iTunes 11.1
calibre 1.4
Marvin 2.75
iOS Reader Applications 1.4.1
Marvin XD 1.2.2
Very happy to say that ALL the apps work well together with functionally that meets my needs.
I plan to stick to the adage: if it ain't broke don't fix it!