The Hidden Key to Harry Potter: Understanding the Meaning, Genius, and Popularity of Joanne Rowling's Harry Potter Novels
Good book - makes a very good and strong case for JK Rowling being a latter-day Inkling, in the mold of CS Lewis and JRR Tolkien. Lots and lots of interesting analysis and material. Suffers from having been a set of lectures, converted to a book (Granger even agrees; in an appendix, he compares books about Harry Potter, including his own, and makes this observation). I think this lectures-become-book leads to some repetitiveness, and overly colloquial text at times.
I'm also skeptical that Rowling (or Lewis or Tolkien) have the degree of deliberateness and precision in use of symbols that Granger credits. But it is all worth reading as fodder for understanding Harry. The speculation about Harry's future is fun, I suppose, but it is already dated; I could do without it.