Time Travel

We've seen the Back to the Future trilogy countless times, but as we were cruising through the channels last night, convincing ourselves there really was nothing on, we stopped to watch a few minutes of Part II.  We noticed a problem - a potential glitch in the space/time continuum.  In this scene, while Marty and Doc are caught up with rescuing Jennifer before she encounters her future self and seals the cataclysmic end of the universe, Old Biff steals the DeLorean and travels back to 1955 to give his younger self the Sports Almanac, thus ensuring his future fortune and altering history.  He then returns the time machine to 2015 before Doc and Marty know it's gone.

When the Dynamic Duo of time travel return to 1985 they land in, not the time period the remember, but an alternate 1985 which has been created by thirty years of Biff manipulating his inside information about the future.  To correct this debacle, Marty suggests they return to 2015 to stop Old Biff from stealing the time machine, but Doc points out that they would be traveling into the future of the alternate timeline so their only option is to travel backwards to a point in 1955 before the timeline skewed and stop Young Biff from receiving the almanac.  Perfectly logical so far, right?

So here's our question:  Once Old Biff places the almanac into Young Biff's hand, the future has effectively been altered, right?  So how did Old Biff return to the same future from which he departed?  Why didn't he travel into the alternate future?

Pondering this conundrum lead to thoughts of other movies one or both of us have  enjoyed over the years which contained elements of time travel:  Field of Dreams, The Final Countdown, Somewhere in Time, Star Trek: The Voyage Home, Kate and Leopold, Groundhog Day.  Of course the next logical step was to reminisce about time travel books we've read.  Here's a few of our favorites:

We know there are lots of other's out there - Diana Gabaldon's Outlander series, The Time Traveler's Wife by Audrey Niffenegger, and the classic Time Machine by H.G. Wells - but we need some recommendations.  What's your favorite time travel movie or book?  Give us some ideas and we'll try to read one of them together and report back.