I've searched around and deduced that adding custom fonts to the BeBook Club S involves a few things:
-Create a folder called "fonts" on the SD card. Some sources say it should be inside another folder called "crengine".
-Place TTF files inside this folder.
-Title fonts after a particular pattern (uniform case, specific naming for bold/italic).
-Fonts may or may not work on MOBI and EPUB files, something to do with fonts being embedded in the files themselves (maybe Calibre can edit this?).

I've tried creating the folder and naming the fonts, but I can't seem to find where to set them (if they appear at all). Accessing "Settings" followed by "Default Font" does not show the new fonts I added. I've tried shutting down and restarting the reader after the fonts went in and still have not met with any results.

Can someone confirm whether it is indeed possible to add fonts to, specifically, the Club S reader and, if so, exactly how to do so and how to subsequently utilize the fonts?

Thanks!
The "fonts" folder on the SD card should not be in another folder. Also make sure that the folder name is in lower case letters (so "fonts" is right but "Fonts" is wrong). Copy your ttf files directly into this folder (so no further subfolders). Then it should work.
The user manual tells you how to set fonts. Your fonts will automatically be added to the list; there is nothing else you need to do.
Thanks. Unfortunately I don't have (and cannot seem to find an electronic copy of) the manual for this device.

I've verified that there is a folder titled "fonts", all lowercase, in the root directory of the SD card. Inside are 4 font files, their own names (BOOKERLY.ttf and bold/italic variants) all in uppercase.

When I open "Settings" from the main menu and choose "Default Font", the list goes from "Bitstream Vera Serif" to "Courier Std" with no Bookerly entries.

I've tried with upper- and lowercase font names. Do you have any ideas what I might be doing wrong (still), or a link to a copy of the manual?

Thanks.
Update: I'm seeing Bookerly in the list of fonts when viewing a .txt file, but trying to select it causes the device to pause for a few seconds and drop me back to the library.
The fact that you see Bookerly in the list indicates that you got the folder name right.
There may be something wrong with the ttf file. Try downloading a free font, for example from Google fonts, and see if you get that working.