Never buy Olive Tree books via the apps on smart phones. Do it through the main website, otherwise, there may not be any refund, the books would be more expensive, etc.
As for general resources, OliveTree's materials are a lot of them miscategorized, ie. book as commentaries, etc. Granted, there are some good materials as well, but at a price.
If it is not for my smartphone, I would stayed with e-sword. Cheaper and lots of free modules. Logos' Bible app for smartphones could prove more useful as well.