So this is what I remember, and it's very specific, but even so I can't seem to track down the novel:
a man discovers a portable time travel device that he can carry around with him. He gets caught in the field and watches civilization after civilization rise and fall and die - because he's in the field, and radio waves transform to the visible spectrum because of that, so he can watch civilizations rise and fall while he's within the field.
eventually the device runs out of power and he's caught far, far in the future - a future where many civilizations have risen and fallen, so much so that previous civilizations realize that human civilization is cyclical and they leave behind nearly-indestructible plinths etched with basic science to help future civilizations along.
he ends up in a minor civilization that's doing it's best to avoid violent conquest by a global super power, they reverse engineer his time travel tech, and do everything they can to prevent conquest. Everything they try fails - and they try everything they can think of.
he falls in love with the daughter of the leader of the minor civilization, but she ends up dying when her flyer is shot down. When they use the device no one remembers her, not even her father, and the time traveler is heart-broken because he's the only one who remembers she even lived. Everyone else tells him to 'move on' but he can't.
The reason I'm looking for this book is because I can remember a whole bunch of specific details about it (e.g., they tried to assassinate the 'great leader' of their nation, and that only ended in nuclear apocalypse), but I can't remember the ending. I want to track down the book so I can read how it ended. I don't own it, I want to own it, so I can read the ending and finally know how everything goes. Wish I'd bought the thing 20 years ago, but I didn't, and now here I am.