But there are exceptions to the unhappy marriage rule, the union of Kitty and Levin in “Anna Karenina,” for one, and to me even more satisfying, the marriage of Plantagenet Palliser and Lady Glencora in Trollope’s Palliser novels. Not the least of the appeal of this fictional marriage is that it takes place over some 20 years or so in six different novels (seven if you count “The Small House at Allington,” though it’s strictly speaking one of Trollope’s Barsetshire novels, not a Palliser book), in which Plantagenet and Glencora are sometimes major characters, sometimes minor ones. The books are like a mini-series (and, indeed, became a famous BBC 26-parter in the early ’70s): We get to watch the characters evolve, grow old and surprise themselves.

Blond, clever, charming, witty, Glencora is easily the most beguiling of Trollope’s heroines. Trollope as much as admitted that he was in love with her himself. But except for his tremendous fortune, there’s nothing lovable about Plantagenet, who actually prides himself on being dull. All he cares for is politics and a mind-numbing scheme for switching British currency to the decimal system. Glencora is bullied into marrying him by her guardians, who fear that she is about to run away with a handsome scapegrace named Burgo Fitzgerald.

Trollope, a much sexier writer than he is given credit for, leaves no doubt that the attraction between Burgo and Glencora is powerful and erotic, or that Glencora’s feelings for her husband never heat up to the same degree. Marriage, both here and in his other books, is not necessarily ecstatic but, rather — when it works — a shifting arrangement of trade-offs. The relationship of Plantagenet and Glencora begins in antagonism and misunderstanding and gradually, over the course of thousands of pages, warms to understanding and to a fond attachment that is erotic in its own, understated way. She thaws him out and he loves her for it.

They also quarrel and reconcile a lot. The marriage’s most volatile period comes in the penultimate book of the series, in which Plantagenet, though he is really too thin-skinned and self-righteous for the job, becomes prime minister. Newly energized, Glencora throws herself into advancing his career and becomes a kind of 19th-century Pamela Harriman, meddling in elections, spending her own considerable fortune on parties, dinners, receptions. Plantagenet hates the vulgarity of it all but gets swept up all the same. You feel on both sides (and hers especially) a growing ardor, a new understanding of what they can do for each other.

And then, shockingly, it’s over. Too stiff-backed to compromise, Plantagenet loses an election and the premiership. He and Glencora leave for Europe to lick their wounds, and on the very first page of the next, and last, volume, Glencora is dead. Plantagenet, though in his way just as repressed and socially hidebound as Walter Bridge, finds himself even more in love. “He had at times been inclined to think that in the exuberance of her spirits she had been a trouble rather than a support to him,” Trollope writes. “But now it was as though all outside appliances were taken away from him. There was no one of whom he could ask a question.”