I don't know if it

I don't mean to blast the book. It was well worth reading (enough so that I handed it off to Abbie, which I don't do lightly) and I think the mystery at its heart is intriguing. Kids seem to love it, which is testimony enough. And who wouldn't welcome a book that pays homage to A Wrinkle in Time?
[Spoiler alert: I'm about to address a key plot premise.] While I didn't think about it as I read the book, I have questions about time travel. I am bothered by the logic of it -- what philosophers and scientists call the grandfather paradox. Which is simply the question of how can you travel back in time and change events without changing the future from which you came? (The name -- don't you love it when you find there's a name for some vague thought you've had? -- comes from the hypothetical example of a time traveler who goes into the past and kills his own grandfather before he fathers the time traveler's parent, making the existence of the time traveler impossible. Meaning he can't go back in time and kill his grandfather. If you think about that long enough, your head will spin.)
One theory is that when a time traveler changes the past, he or she creates an alternate universe. If I get it -- and I'm not at all sure I do -- both realities exist. I don't find that explanation very satisfying. Especially when the whole point of the time travel was to undo something that happened.
I need to read more about time travel, I guess. Anybody know a good place to start? Most of what I found online was either way complicated or too shallow.
And with the movie coming out next month of The Time Traveler's Wife by Audrey Niffenegger -- one of my favorite novels on the past few years -- maybe I need to understand.
PS: If you've read this far, please also read this related post.