Logos Bible Software has (much like everyone else) been putting out apps for iPhone/iPad, android, and one even available over the internet. Not everything they sell is available via these apps (due to licensing issues), but a lot of their products are.
Since they're becoming more useful, I want to share a couple of good packages with you. (Note, I'm not making a dime off this.)
Rejoice Christian Software has the Essential IVP Ref. Collection 3.0 for $89.95 (for a limited time), the Norman Geisler Apologetics Library for $29.95, and the Essential Bible Study Library for $12.95 among others. There are a lot of good, useful books in these collections that you can now read on your pc, smartphone, or tablet. I think they're a good investment for your Bible study library.