I read Dan Simmons' Hyperion at University, where it was actually a set text for the class I was taking (Science Fiction and Fantasy literature; of course, my termpaper was on Tolkien's language creations). Most of the other books I read for that class passed me by, but Hyperion has stayed with me ever since. I don't remember much of the story, or that of the sequels I read afterwards, but this bit is still clear in my memory.
Hyperion follows the tradition of the Canterbury Tales - seven pilgrims travel to the planet Hyperion, to the Valley of the Time Tombs, to seek the monstrous creature named the Shrike. One of them is an elderly scholar, Sol Weintraub, with a baby. If you want to read the story, you'd better skip the following.
Sol's daughter Rachel is an archaeologist. Many years ago, she researched the Time Tombs herself, until a disturbance happened. Recovering in the hospital, she is thought to have suffered some sort of memory loss. It is only later that she is revealed to age backwards from the moment of her accident - each morning, she wakes up without any memory of what has happened in the Tombs, or in as much time before the disturbance as has passed since it happened.
Her story is the most heart-breaking thing I have ever read. At some point, she doesn't recognise her fiancé, and whatever he manages to tell her is lost again when she wakes up the next day. At first, they try to brief her every morning, until she asks them to stop, and live the rest of her life not knowing what happened.
Her parents take her back home, and each day for the next two decades, they pretend it is that day two months ago, five years ago, fifteen years ago. Through all this time, there is one family phrase that she holds on to - someone will say, "See you later, alligator", to which she replies, "In a while, crocodile." Even as a small child, she still remembers it, until that passage in the book that still haunts me - Sol, now old enough to be her grandfather, takes her to bed and says his phrase, and she just laughs. With a chill, he tells her how to respond. "While, 'acadile," she replies.
"In the morning, she'd forgotten."
She is the baby that Sol has brought on the pilgrimage.