This is another great book for beginners: http://www.amazon.com/gp/product/143023024X/ref=s9_simh_gw_p14_d0_i1?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-2&pf_rd_r=0M22VGVRBQH488KCHM83&pf_rd_t=101&pf_rd_p=470938631&pf_rd_i=507846 It's really up to date; it came out today.
About the app idea, you have a couple options: create something totally new, reinvent something (like Angry Birds), or to create an app that is so good it beats everything in it's class. I haven't released an app (yet), but the last one is definitely the hardest. I would try with the 2nd one because it is really hard to think of something that hasn't been done before.
Other than that, I can't help you with the app idea. :(
This is a pretty great book for iphone developments:
It uses xcode3 rather than xcode4 but its basically the same thing.
For objective-c projects in xcode you might as well be inheriting from Foundation or NSObject. Everything for mac or cocoa touch inherits from NSObject at some point.
Lastly I as well would choose to use xcode for anything objective-c. But another great IDE for many other languages would TextWrangler.
Beginning iPhone 4 Development: Exploring the iOS SDK --> http://www.amazon.com/Beginning-iPhone-Development-Exploring-iOS/dp/143023024X/ref=sr_1_3?s=books&ie=UTF8&qid=1290354782&sr=1-3
helped me very much and is currently 30% off ;)