I’d never heard of the painting Christina’s World but it is an otherwise well known work by American artist Andrew Wyeth; Christina Baker Kline uses the painting as a launching point for her novel A Piece of the World which does an excellent job blending the lives of real people into an enjoyable piece of fiction.
“Later he told me that he’d been afraid to show me the painting. He thought I wouldn’t like the way he portrayed me: dragging myself across the field, fingers clutching dirt, my legs twisted behind. The arid moonscape of wheatgrass and timothy. That dilapidated house in the distance, looming up like a secret that won’t stay hidden.”
The story of the Olsons and Andy Wyeth is told with interspersed flash backs to Christina’s childhood and early adulthood. In the early 1900s Christina Olson is growing up on her family’s farm, a legacy from when her ancestors, the Hathornes, who moved to Maine in the late 1800s. As a child Christina suffers from severe pain in her arms and legs that eventually leads to crippling (it sounds like Polio in the book but according to the Internet it was most likely undiagnosed Charcot- Marie- Tooth disease). 
As a young woman she gets her heart broken; this along with her disability leads her to a quiet life living on the family farm with her younger brother, Als, as her main companion. Until one day when a young girl, Betsy, befriends a middle aged Christina. Years later, when Betsy meets her future husband, Andy, she brings him to the Olson farm and he is immediately taken by the sprawling homestead. Andy is the son of a famous artist who feels the urge to create art himself; he begins spending summers painting in the Olson house with Christina and Al occasional posing. This friendship leads to Christina’s World.
I don’t know how much of A Piece of the World is pure fiction or conjecture but it appears to be a well researched novel with roots in reality and an overall enjoyable read.