The Ride of Her Life: The True Story of a Woman, Her Horse, and Their Last Chance Journey Across America by Elizabeth Letts - In 1954, 63 year old Annie Wilkins is told by her doctor she has maybe 2 years to live. .
So...not going to lie, but I read this as "Annie Wilkes" (and Maine, even if Misery takes place in Colorado), and...that was not the book I would expect to be about Annie Wilkes (the Kathy Bates character in the movie)! Might have to track down the original book written by Wilkins, not Wilkes...