I guess my root problem really isn't answered. I still can't build books with a horizontal rule in them. Do you have any suggestions for how to do that?
I've installed the latest version of Pandoc (2.9) on my mac (using homebrew to do so).
However, now that I'm running latest version, when I attempt to build the demo book, I get the following error:
pandoc-citeproc: Error in $: Incompatible API versions: encoded with [1,20] but attempted to decode with [1,17,0,4].
CallStack (from HasCallStack):
error, called at ./Text/Pandoc/JSON.hs:113:48 in pandoc-types-1.17.0.4-JUbKfxLZkCY3AkhtmFEGjK:Text.Pandoc.JSON
Error running filter /usr/local/bin/pandoc-citeproc:
Filter returned error status 1
Error: pandoc document conversion failed with error 83
Execution halted
Exited with status 1.
I figured maybe I needed to update pandoc-citeproc to the latest version, too, so I installed that (version 0.16.4) with Brew as well. That doesn't seem to help, though.
It appears maybe I need to install the latest version of pandoc-types? But there is no brew recipe for this, so I don't really know how to install it. Plus, I don't know if that will fix the problem.
Can anybody help me build a book with a simple horizontal rule in it?
Like that ^ ! 
Thanks in advance.