I haven't read the books, but even if I had, I might still have a few issues with the story. First of all, it seems silly that this whole epic fight over saving the universe comes down to a mere gun fight. Also, how come the "gunslinger" who comes from another world uses the exact same type of bullets we use here in earth? Seems unlikely. Also seems unlikely that these other worlds would even be exactly like earthly humans.
Furthermore, I hope the books included more lead characters that are female to make it balanced, because the movie has none. It's pretty much all an all-male cast.
Lastly, I hope the book explains more than the movie did, because we are just supposed to buy that the tower protects against dark forces, but they don't explains why or how.
However, the acting was pretty good, especially by the lead boy.