A Year Without Autumn, by Liz Kessler

One day, when Jenni is on her way to her best friend Autumn's place to go to a horseback riding lesson, she discovers an old elevator and takes it instead of the usual one.  When she arrives a Autumn's apartment, her friend is long gone -- an entire year has passed!  But more than that, a chain of events have been unleashed, causing a serious injury to Autumn's little brother, the loss of Autumn's friendship, the collapse of Jenni's parents' marriage, and grave confusion for amateur time traveler Jenni!  Now, Jenni has to figure out a way to set things right.

Combining a touch of magic and time travel with middle reader standard themes of friendship and family, this story is very approachable.  The flaws are minor and countered by its strengths.  It deftly maneuvers most of the complexities of time travel (although Kessler's insistence on doing so sometimes overextends the authenticity of Jenni's narrative voice in the explanations she provides).  Keeping the focus of the story solidly on Jenni's and Autumn's friendship helps to alleviate the onset of reader boredom.  Somewhat more annoying  are the psychological pep talks that Jenni and Autumn have about grieving.  Again, these are overly complex (most adults would lack the introspection that Jenni's freely espouses) and the discussions didn't drive the story very well.  A side plot about an old woman who has spent her life regretting her own mistakes didn't quite gel either, but serves to illustrate the dangers that Jenni and Autumn face.  So, in spite of the tendency towards excessive expository writing, the novel is quite readable.