I was in a small local health food store recently, and all of their books were 75% off.
I found these 2 books:
1) Alzheimer's Disease: What If There Was a Cure?: The Story of Ketones, by Mary T. Newport, from 2013.
I got it for $4.74
2) The Edge Effect: Achieve Total Health and Longevity with the Balanced Brain Advantage, by Eric R. Braverman, from 2005.
I got it for $2.74
Looking forward to reading these! The first one I listed has really good Amazon reviews, the second one seems to be a little more controversial. I'm not recommending them because I haven't read them yet, just wanted to share my cheap book finds.