I'm about halfway through Diana Gabaldon's Drums of Autumn (3rd in a series of very FAT books, all about one couple) and the romance between Jamie and Claire is just as fresh as in Outlander.
How can this be? Kinda makes me ponder sexual tension. Most of the time it's really hard to keep a reader's interest once the H and h proclaim love and hit their happily-ever-after. How can daily drudgery and dialogue between these two be so intrinsically interesting?
Well, for one thing, their adventures are pretty darn believable, yet exciting. And you can see their feelings for one another. It's obvious by the characters' actions and dialogue.
And now I've got a new book in the series to collect...