Considerations

See About the Ink
Toilet paper comes off roll bottom first. It would have to be turned up to read text in sequence.
Publishing an entire book on TP would not be as economical as publishing it in regular format.
Books on TP would need to be limited to one user. Multiple-bathroom users would have to keep their rolls of TP separate somehow (multiple roller dispensers, one for each person, and one for guests?).
Some people may have an aversion to using good reading material to wipe their bottom side.

See also