Besides the Core Data book, I have also borrowed two other iPhone books for my bedtime reading.

Programming the iPhone User Experience expands on the required Apple HIG.  Good read, especially on parts of iPhone UI that you seldom use.

The iPhone Developer’s Cookbook has lots of sample code that I may use later.  Wish I had this book earlier.  Although a bit outdated, but still great as a reference.

OK and no point buying these books as you can get them from the library when you need them.