I have a definite penchant for time travel. I like all different kinds - time loops, time machines, paranormal," parallel timelines, history monks, even "it was all a dream" if we get a satisfying view of the life changes a character makes because of it.
Time travel does have a central problem though: the more logical and consistent forms of time travel make for the worst story telling, and the best story telling makes for the hardest to swallow time travel mechanisms. Think back to "Back to the Future." It makes no sense for Marty's siblings to be gradually disappearing from the picture. Either what he has done has erased them from the future or it hasn't - half gone make no sense. But it makes for good suspense and drives the story, so we swallow it.
The time travel mechanism here is similar - good for suspense; not so good for making sense. Basically, the time line protects itself by erasing (killing) time travelers from the future who are going to change the timeline beyond its ability to repair itself. This is actually very clever and makes a lot of sense. Except the timeline doesn't just kill the traveller right before they are about to break things. It starts trying to kill them when they have stayed too long, and escalates its efforts until it succeeds (or the traveller goes back to their own time). Thus, our time traveller, Reegan, and his adversaries face a bunch of weird accidents that injure or kill them and they know more are coming until they are dead or leave. Which, no. I can buy the timeline protecting itself, but this is more like an allergic response - it wouldn't work to protect the timeline, except on average. But OK - I had to swallow it in order to enjoy the story, so I did.
The premise is pretty exciting, and in many places the story is too. Reegan loses a client (Sylvia) on a trip to the past and must travel back to get her. It becomes apparent that she has disappeared on purpose and doesn’t know that the timeline will kill her in short order. Once Reegan is in the past, he realizes that others have followed him, to capture Sylvia and kill Reegan. So he has two enemies – the timeline and the other time travellers. He has one ally – a private detective named Saul. They have to find Sylvia, avoid the other time travellers and get Reegan and Sylvia back to their own time before they are killed. Since this is a romance, Reegan and Saul are increasingly unhappy about the fact that Reegan has to leave.
The views of Reegan’s time – a few hundred years in the future – are intriguing. It is kind of a combination of utopian and dystopian. Everyone has plenty to eat, but the food is mass produced and boring. Everyone is educated, but some animals are obviously much more equal than other. Sylvia and her husband are trying to open up opportunities for the lower classes (from which both she and Reegan came). Time travel tourism exists, obviously, and crime investigation is completely changed, since lies are easily detected.
So why only 3.5 stars? The characters, and the near-future where Saul lives and Sylvia and Reegan visit. The characters are drippy. Saul is a mopey alcoholic. Reegan is mopey and not very good at his job, either as a historian (he makes constant mistakes about Saul’s era) or as a time travel tourism leader (he keeps losing people). Sylvia is mopey and dumb (how could she not know the timeline would kill her when it was taught in elementary school and she researched time travel for months). I can swallow insta-love when the main characters are in a suspenseful, high-pressure situation but Saul and Reegan didn’t make me want to believe. They were too drippy and incompetent.
The other thing that kept jarring me out of the story was Saul’s time, the year 2020. Saul was a cop who was outed as gay and is shunned by every single cop he ever worked with. Not just ignored, but bullied and black-balled. In the year 2020. In the D.C. police force. That plot device already doesn’t work now, except maybe in a small southern town. No, not even there – some of his colleagues would act like they lived in the 21st century and not in the 1950s. I think the author was going for a noir feel, but if so she should have set the action in a much earlier time. Nothing about 2020 felt like it was in the future or even in the present. The characters drove around in cars, ate bar food, rang doorbells, and used projectile guns. The treatment for alcoholism consisted solely of white-knuckled cold turkey.
I listened to the audiobook of this story, and I think I would have enjoyed it more on my Kindle. There was a lot of repetition and unnecessarily drawn out conversations that I would have skimmed. For example, Sylvia gives Saul two lectures about abusive relationships, and I definitely would have skipped the second. And many of the main character’s ruminations on guilt were skim-worthy.
 Time loops: the classic is Groundhog Day. An M/M example is “Rinse and Repeat” by Amberly Smith
 One of my favorite time machine stories is The Stainless Steel Rat Saves the World by Harry Harrison. This book, “Paradox Lost” is also a time machine book.
 “Trick of Time” by J.L. Merrow and “A Token of Time” by Ethan Day are two good M/M examples.
 Parallel timelines show up a lot in movies: “Days of Future Past” and “The Terminator.”
 The History Monks are from Terry Pratchett’s Diskworld books, notably “Thief of Time.”
 “Turn” by Sara’s Girl has this trope but is still awesome.