I'll limit Book Review No. 32 to some of the larger ideas, rather than spoil the story.
First, the plot might depend on parallel universes, as well as time travel. I once heard a cosmologist use Flatland as a way of raising the possibility. Imagine a compact-disc stack loader with arbitrarily many discs loaded into it. Each disc represents a universe. A Flatlander on any disc can go about his business (that's deliberate, the ladies are line around) without any interaction or even awareness of the behavior of Flatlanders on any of the other discs, and with arbitrarily many discs, the existence of two universes with almost identical populations and histories is almost certain.
Second, in a higher-dimensional non-Euclidean space, the possibility of interference by one universe in the space-time-thought continuum of another might make more sense than the rubbing of two discs together when the stack loader wobbles. And thus the operator of a downscale diner in a part of Maine that is no longer paper, potatoes, and textiles but not yet yuppie getaway country blunders into an overlap of universes. First, he exploits the overlap to get some incredibly cheap aged beef for his diner. When he notices that each trip takes him away from what he perceives as real time for two minutes, no matter how long he is away, he starts thinking about stopping Lee Oswald.
Third, instead of thinking about parallel universes and the space-time continuum, contemplate the existence of Norns, who "spin and weave and cut each thread on the web of life and the fate of every person and creature", who in the singing the executive summary of Rheingold and Walküre and Siegfried at the beginning of Götterdämmerung, break the thread, in order that the Gods be consumed by fire and the Rheintochter retrieve their gold four hours later:
Now put yourself in the position of Norns keeping track of all these threads, which have to be re-woven every time this diner operator turns up to buy another load of aged beef, and then he goes on to stop Oswald.
Fourth, consider the Liberal Pieties. Had JFK lived, he would have backed off in Vietnam, rather than doubling down as LBJ did. Butter, but fewer guns. Imagine no social polarization. Thus a Mainer who shares Stephen King's worldview and has access to what he sees as a time-portal is likely to want to implement those pieties.
To find out whether the Liberal Faith was rewarded, read the book.
(Cross-posted to 50 Book Challenge.)