iPhone chess apps where you can learn while offline

I want to spend my commutes on the subway with a chess tutorial app.  I currently use the chess.com app, and it's great, but most of the features require you to be online.  Any recommendations for other apps? Thanks.

TacticTrainer works offline

I use tChessPro and SmallFish - both can work with PGN files. I load them with PGN databases of tactical or endgame problems and play those positions against an engine.

PHudson, can the lessons in the app be used offline too? If so how? Is there a 'make available offline' option I need to switch on?

I was referring to a paid app that is separate from chess.com. I don't believe the chess.com app works offline.

http://www.gambitchess.com/

https://sites.google.com/site/chessresources2share/home/books-in-chessbase-collection

You can always google for more. Many collection are in Chessbase format - then you'll have to convert them to PGN.