A player can use a device or reed an ebook
About how to play against a 3 bets or how to play against poker players?
In a cash game session or tournament?
TDA Rules prohibit talking on a cell phone during play. Use of other electronic devices is up to the individual house. Personally, I do not believe a player should be allowed to use any type of device during play. However that is not the position of the TDA.
My suggestion is that your house rule should prohibit the use of any type of device, during play of a hand, that could be used to give the player any advantage during a hand that player is involved with.
In prior years, the TDA rule prohibited use of any electronic devices, but there arose such a clamor from players that needed to post to Facebook or Twitter that the TDA rule was changed.
Chet
I don't think any of the TDA members actually like players to use electronic devices at the table. Unfortunately as electronic devices become even more a staple in people's lives, it is like trying to fight off an avalanche. :o