To Say Nothing of the Dog, by Connie Willis
What's not to love about a book that manages to combine time travel, Victorian house parties, British mystery authors, boating, and metaphysical ponderings on the intersection of free will, chance, and design? This book was pure fun to read, yet with enough Big Ideas to keep it from being fluffy.
The description of time lag, a disorder caused by too much time travel too close together, sounds remarkably similar to postpartum sleep deprivation (including the sentimentality--never watch Little Women at such a time), so I identified quite strongly and persevered even though the narrator was so confused at the beginning of the book that it was hard to tell what was going on. It does all clear up eventually, and most satisfactorily.
I appreciated the way the Victorian era and morals were treated, with respect but not slavish devotion (for those not timelagged, anyway), and the way religion was handled in a novel primarily set in a post-religious time.
Now I'm eager to try more books by this author, as well as the original Three Men in a Boat, To Say Nothing of the Dog. Many thanks to Erika for the recommendation; this is not a book which I would have stumbled across on my own.