Hi Jordan. Please do read this entire post, as the early parts may come off the wrong way otherwise. I promise there's a reason why I made my original post and this one.
I know you're the webmaster here, but I'm also not a random noob. I spent almost 20 years between consulting, contracting, and working directly for Microsoft.
I say that not as an ego stroke or an attempt at logical fallacy via appeal to authority. I state it merely to express that I'm quite technically capable as well.
I was seeing comments on the Facebook Constant Reader group from folks having issues accessing the content.
I am referring to the link entitled "Download The Bill Hodges Concordance Now" as you've stated, not the link entitled "Concordance" which is just a pointer to the html page itself. I know that the actual file stored is an ePub and that the hover link correctly identifies it as such.
However, when attempting to download the file, even if the 'Save Target/File As' option is selected the file may still attempt to save as a .html file that will have to have the file name extensions manually changed to .ePub after the fact.
This is the case in Microsoft Edge on Windows 10 devices for example. A direct download doesn't recognize the .ePub extension and automatically saves it as a .html file. Using the 'Save Target As' also saves it as a .html file that has to be manually renamed after the download.
IE11 on Windows 7 and 8 also has the same behavior, with the exception of being able to rename the file extension in the download manager after selecting to save as 'all files'.
The issue doesn't appear to occur in Firefox or Cnrome on Windows desktops, which is why you likely haven't seen it as I'm guessing you use Chrome.
For mobile devices attempting to download the file it gets a bit trickier as it seems Webkit-based browsers (pretty much the default browser on all Apple devices) seem to also try to automatically treat the extension as .html instead of passing along the .ePub extension properly.
I've seen this happen on a few websites where there's a misconfiguration in how certain filetypes are treated. So instead of just passing along the raw file it parses and treats it as a different filetype at download.
I used to know the fix for this issue back when I worked on more web development based items, but sadly have forgotten the little codehack that tells the site to just send all download attempts as an 'all files' item so that they don't run into this issue.
Figuring you're the webmaster I figure there's likely a good chance you'll know what I'm referencing and hopefully know the fix so that future downloads don't run into this.
(Please don't take this as my trying to tell you your job or trying to be pedantic or the like. Too often my posts are taken that way because I tend to be fairly dry in how I write on message boards.)