In World Enough and Time
Haskell Yngren and Jennifer Jessup, with Jennifer's long-dead parents leading the way, travel back in time to witness historical events and sideways through alternate realities (resident in constantly dividing and diverging time streams) to see how things might have turned out, if only . . .
My theory of multi-dimensional time, upon which I constructed my story, is consistent with the Many Worlds Interpretation of quantum mechanics. My grand daughter, in junior high school, having read the book, now has a working knowledge of the theory.