This is marketed as the ultimate time travel novel and I don't even think that's an exaggeration: this isn't about visiting the future or the past, it's 100% about time travel and what it does to a person. It's a psychological look at what that amount of power, loneliness and a mostly consequence free life lead to, while also deeply exploring a time travel theory that isn't used as much as others (and might even have been completely new at the time). It explains a lot about it really well, basically everything you need to know for it to make sense is at least implied in a way, but it also leaves enough to the readers imagination.
This book gave me a lot to think while sometimes also being funny or even totally weird (I mean it's from the 70's, what did I expect?).
Not everything has aged perfectly, but me loving time travel stories, this gave me everything I could have hoped for.