I've just finished reading 'We Are Not Ourselves' by Matthew Thomas. It's one of the best books I've read for a long time.
One of the main characters develops Alzheimer's - a bit of a spoiler, I know, but that's the reason I'm recommending it here.
I found it absolutely authentic in its portrayal of the disease and its effects on the lives of others. I don't know of any other novels that have done this but I expect there are some.
I didn't know about the focus on Alzheimer's when I started the novel and wouldn't necessarily have chosen to read such a novel if I had known but I'm so glad I did.