This is an entertaining and enjoyable time travel novel with good characterisation. It deals with "what if" in a similar sort of way to the movie "Frequency" (the one with Dennis Quaid), although it pre-dates the movie by several years. The science bits are well thought out.
This excellent 1980 hard science fiction novel is somewhat like a much more positive and upbeat version of "The butterfly effect" except that the characters are trying to quite literally save the world from imminent destruction instead of making alterations in individual lives.
Of all the dozens of novels I have read which include either time travel or other speculations about the nature of time, "Thrice apon a time" includes far and away the most imaginative and brilliantly explained picture of how a universe in which you actually can change the past might work.
Every single other novel I have read or film I have seen in which the possibility of changing the past comes up is a variation of one of the following views
1) you can't do it at all because the past has already happened and may not be altered
2) you absolutely must not attempt to do it because it will destroy your universe or cause a huge catastrophe such as a split in the timestream
3) you should not attempt to do it because there are likely to be unpredictable side effects which may be very harmful.
The scientists who are the central characters of "Thrice apon a time" discover an unusual side effect of certain reactions - a small amount of evergy is sent backwards in time. They build a machine to measure the effects when that energy arrives in the past - and then find that it can be used like a receiver for messages which can be sent backwards in time. E.g. they can use the effects which their machine measures to send messages back to their earlier selves at any time after the machine was built.
Being very aware of the potential threats described in 2) and 3) above, our heroes initially have no intention of using the machine for any significant communication. Until it turns out that a new experimental reactor which was used for the first time a few days after they built the machine has produced catastrophic effects.
So catastrophic, in fact, that trying to change the past may be their least bad option, even though it means that their present selves and things which are very valuable to them will be erased ...