Is this book only for Mormons?

No.  It's not.  In fact, I've had some friends who are not Mormons read and like it.  The story is about active, believing Mormons and so, consequently, some of the cultural and religious assumptions are uniquely Mormon.  I'd compare it to the books I read by Chaim Potok, like The Chosen, for example.  It's about Orthodox Jews, but that doesn't mean only Orthodox Jews can read it. The book is informed by a faith in Jesus Christ, but, like the Bible, you could read it for religious reasons, or you could read it as literature or out of curiosity. 

Like other groups with a shared culture, Mormons do use of a lot of terms and jargon that might not be familiar.  So, I've prepared a short glossary to explain some of those terms, beliefs, and practices.  You can get it here.