Yes, it is the language used to create iPhone applications.
Yes, the language is probably becoming more popular as more developers learn it, in order to create iPhone applications.
Since iPhone applications are still relatively new, and haven't hit their stride yet, book sales will increase rather than staying steady.
Ruby on the other hand has been "popular" for at least two years now, and it's natural to see a "decline" in book sales, even though the number of developers entering the ruby world may remain the same.
The other thing to keep in mind is that Ruby is relatively easy to pick-up and run with, while Objective-C is, uh, a bit more "complex" (for lack of a better word).
