Question:
Answer:
As far as I know, no universal rule has been issued by the Church, so it if the local bishops conference has not prohibited its use (as the bishops in New Zealand did), and the appropriate liturgical texts are legally available on the tablet device, then it would technically be an option.
The Church usually uses blessed objects for Mass that cannot be used for other purposes. So it would seem a bit odd that an object used for answering emails would also be used to say Mass.
However, if for some reason the proper liturgical books have gone missing or were ruined, it would seem reasonable to use an electronic device for immediate and temporary use. I could also envision an electronic device that is used only for Mass that is carried by a priest who travels between assignments that require him to bring his own liturgical books.