You didn't do nothing wrong.
Sideloading books is easy as you mentioned. I always send mine by uploading then through the
Send to Kindle app which is the same as emailing the book. I also sideload by using Dropbox to download the book then putting the book in the Documents or Books folder and restart the Fire for them to show up.
USB transfer shouldn't need a restart for a book to show up.
Just saw it's a prc file. Rename it to mobi and see if that fixes it. If it does this on mobi extension books then do the factory reset otherwise it's that the Kindle Fire doesn't know what to do with a PRC book.