I read it in the 90's but it was probably much older, it was a young adult fantasy book. The cover was blue I think, and had the blonde girl on a horse and maybe next to a mountain kinda small and to the side, and had a larger illustration of the cat-like elf guy's face and big eyes.
The main character was a young blonde human girl who I think has to flee her home (they may have been nobles who were overthrown) and she goes with I think a maid, and she flees to a distant relative's home who are racist against these cat-like underground-dwelling elf people. She ends up saving/treating a wounded elf guy, and also somewhere along the way became friends with this male giant with a pet spider named spinner or weaver or something like that who could spin healing web bandages (seems like the giant was a shapeshifter, too).
She ends up becoming somewhat friends/subtly romantic inclinations with the elf guy (his name may have been Martin...he says "Sa!" as a curse word and hates horses) however, her helping him gets him in trouble with his own people and he's betrayed by either his best friend (who is like, the prince of his people) or another one of the jealous "blood brother warrior" types who wants to be the prince's bestie instead. Main chick and the giant have to save him, but along the way the giant's story gets interesting as he is the last of his race...awake? He was a baby when all the other giant people went to sleep or disappeared or something, but they end up finding a door underground that her or his magic bracelet thing opens. But she goes to save the elf guy who has become an outcast. They've shaved his fur (these elves had short fuzz all over and large feline eyes) and caged him so people can shame him and spit on him and throw rocks. So she goes before the king and kneels to beg for him to be freed and ends up fulfilling some prophesy about the sun kneeling before the mountain....or the moon or something along those prophetic lines.
Seems like the title MAY have had the word "Shadow" in it...but that could just be the crazy talking. Because it also seems like maybe it had "children" in the title too, maybe referring to the three races in the book (giant, human, elf).