Quite a few of the characters on Game of Thrones have been drastically altered from their book counterparts... and that's not always a bad thing. We can't cram in every one of the several hundred plot-lines, we can't keep track of all those Greyjoy uncles, and we can't reasonably expect Peter Dinklage to act without a nose.
But boy have those TV writers been hard on Sansa Stark.
Don't get me wrong, Sansa's life is terrible any way you slice it. She starts out as a sweet, slightly spoiled, traditionally feminine little girl who enjoys singing and sewing and the Westeros equivalent of Disney princesses fan blogs. Then, in both the book and the show, readers see her traumatized by the death of her father. She's abused by her "beloved" Joffrey, beaten by his guards, forcibly married to Tyrion, and dragged off to the Eyrie by Creepy Uncle Littlefinger, who kisses her before shoving her aunt out a window.
That's where things starts to diverge. In the show, Littlefinger marries Sansa off to the monstrous Ramsay Bolton, who likes to feed women to his dogs for fun. With Sansa as the only living Stark (as far as anyone knows), this gives the Boltons legal claim to Winterfell. But this, of course, is not so great for Sansa.
TV Sansa is repeatedly raped by her new husband. Finally, with the help of a tortured Theon Greyjoy, she's able to escape, make it to Jon Snow at the Wall, and take back Winterfell with the help of Creepy Uncle Littlefinger and his buddies in the Vale.
(The show also takes this opportunity to dispose of lil' baby Rickon Stark, because they apparently don't have the budget for his unicorn-riding cannibal subplot.)
Sansa feeds Ramsay to his own dogs (nice), and takes up the mantle of Lady of Winterfell, even though she is now mistrustful and goth in a new-Taylor-Swift kind of way. She's weird around Jon and the newly returned Arya, until she finally decides to kill Littlefinger in a mildly contrived, deeply satisfying reversal.
In the books, she kind of just stays at the Eyrie.
Her BFF Jeyne Pool is the one married to Ramsay, in the guise of Arya Stark. Sansa remains in the Vale after the murder of her aunt Lysa, disguised as Alayne Stone, the bastard daughter of Lord Littlefinger (just to make him creepier).
After all those years of looking down on her bastard half-brother Jon, Sansa is forced to live the life of a bastard. She's slowly unlearning all of her courtly prejudices. She's also being set up to marry Harrold Hardyng, a distant cousin of the Arryns, who'll become the heir to the Vale should something... happen to sweet, sickly little Robert Arryn.
But Sansa has learned from her time at the Red Keep and from her many previous betrothals and marriages. She's learned from Littlefinger, even though he's objectively the worst. She's become an expert at lying, at acting the role of the sweet, pretty maiden in order to stay alive. Starting with the death of her direwolf, Lady, Sansa has become increasingly aware that the world is unfair. She's grown hardened and tough and disillusioned with life.
Or, as she puts it:
My skin has gone from porcelain, to ivory, to steel.
As the only living Stark without a wolf, Sansa has had to weaponize her extensive knowledge of courtly manners instead.
She's a character of towering strength in both the book and show. But in the book, she's turned her love of romantic stories and her impeccable manners into her arsenal.
In the show, she's yet another instance of a girl who's damaged by a man, and subsequently becomes stronger and grittier and wears all black.
To be fair to the show, their take on Sansa isn't unforgivably bad or evil. It streamlines several disparate plots. Actress Sophie Turner is perfectly cast. It's just a well-worn trope of a sweet innocent girl turned badass, damaged woman. It's sexual assault used as shock value, or as "edgy" character development. How much more of that do we need?
The show has a nasty track record of adding extra scenes of sexual assault to a series that already has plenty of violence in it. It's satisfying to see Sansa get her revenge on Ramsay, but not every character in the story has to have the same trajectory. Feeding people to dogs and having Littlefinger stabbed to death isn't the only way to be strong. There is a version of Sansa who gains a more nuanced worldview without suffering quite so viscerally.
There's a version of Sansa where her main character arc is not revenge, but unlearning her own bigotry. She learns that Joffrey, the handsome prince, is cruel and hateful, but Tyrion, the "imp," is kind and respectful to her. She starts to understand that bastards are unfairly ostracized, and that the highborn lords and ladies are not to be trusted. She's not as badass as her TV counterpart, but she's well on her way to becoming kinder.
As for where she's headed next, we'll just have to wait for the next book.