This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
You say you know Java, JS and XML. IMO, transitioning to Android development will be easier for you. For iPhone development, you will need to learn Objective C and familiarize/verse yourself with Xcode.