Fiction by Lisa Genova.
This book is by the author of Still Alice, the story about the woman with early-onset Alzheimer's disease that was recently made into a Julianne Moore movie. I haven't read that book --in fact, this is the first book by this author I have read-- but I'd imagine it to be pretty similar to this one, judging from the movie. (I don't know if all of Lisa Genova's books are like this. Does she start out by wondering what sort of debilitating disease with which to doom her characters, and then begin writing? Sounds like fun, right?)
Inside the O'Brien's also focuses on a character facing a terrible diagnosis; this time it is Huntington's Disease. (That's not a spoiler, by the way; it's obvious from page one what's happening.) The really sad part is that this disorder is inherited, and so the whole family is directly affected.
Obviously this is not book for you if you are looking for a happy story, but it is amazingly readable and engrossing. It makes you think.