After reading lots of research and many book excerpts, I learned that Foundations of Neuroeconomic Analysis (2010) was the best overview available on how the brain encodes value and makes decisions. But I couldn't find it for free.

Here's another place to look: I downloaded "Foundations of Neuroeconomic Analysis" from library.nu for free just now, as a 488 page PDF file. If you don't create an account, you can search for books and find pictures of the covers, but if you create an account they'll show you links that let you have files for the books. The format varies: usually PDF, sometimes mobi or djvu.