Jude Deveraux has a couple of novels that really nail the time-travel romance thing, where the history isn't just wallpaper. 'A Knight in Shining Armor' is the big one everyone knows—Dougless Montgomery gets dumped and cries on a tomb, and this 16th-century earl, Nicholas Stafford, shows up. What I like is that the history feels messy and real; Nicholas isn't some polished fantasy knight, he’s confused and angry about being in the future. The romance builds from that clash.
Her other one is 'Remembrance', which is part of the Montgomery/Taggert family saga. The setup is a reincarnation story more than a straight time slip, but the past-life sequences in medieval England are extensive and drive the whole plot. It’s less about the mechanics of time travel and more about the soulmate-across-centuries idea, which can feel a bit sweeter and more fated than 'Knight'. Some readers find that more appealing, others think it’s too neat.