Apple Devices on Windows cannot sync books

I have a number of ePubs I've purchased or downloaded from the internet and which are not even available for purchase on Apple's book store. I recently updated to the "Apple Devices" app for attempting to sync content between my devices and my Windows PC and discovered that when I don't have significant connectivity issues (I cannot get my older iPad to list any programs it can transfer files from in the "Files" section), I simply cannot get the "Books" app to show up in the "Files" section so I can transfer my files to my devices. This is, sadly, my primary use case for any syncing mechanism: transferring my books and files from my Windows PC to my Apple devices. Is this an oversight? Am I doing something wrong? I've been acquiring digital books for years now and this seems like an odd regression. I can't help but notice that when I revert to using iTunes, this problem mysteriously vanishes.