ZFT
From Equestripedia, the Archives of Equestria!
ZFT is a top-secret book located in the Dark Magic wing of the Royal Library of Canterlot. It was among the books Sunset Shimmer either read, or was to read.
Behind the scenes
It's possible that "ZFT" may be a reference to "Zerstörung durch Fortschritte der Technologie", often shortened to "ZFT", a terrorist organization from Fringe.
