I think TOS contradicts itself on subspace radio. The Horizon didn't have it yet the treaty that ended the Romulan War was negotiated over subspace radio. Both occurred around the same time. Though I suppose that if the Horizon was a civilian vessel (I don't recall if the designation USS was used in TOS: APotA)then they might not have had subspace radio as that may have been purely military tech at that time.
I'm not sure about this but I think the book seen on Enterprise had a slightly different title than the one seen in A Piece of the Action.
- edit - Just checked Memory Alpha and the books do have different titles. The ENT one says Chicago Gangs while the TOS one is Chicago Mobs. http://en.memory-alpha.org/wiki/Chicago_Gangs