After adding a font with Font Book, is it safe to delete the original file? I often find free download resources for font faces. I usually download the file and then double click ,expand, install with Font Book. It has never been clear to me if Font Book copies the font files to a system folder or if I need to keep the original file. I have encountered some fonts that became corrupt  at a later date and I wasn't sure if that was because the original file couldn't be found. Can anyone verify that it is safe to trash the downloaded file after installation?
The fonts you import to Font Book are imported into the system and Font Book no longer uses the original.

Yes, you are safe to delete the original fonts you have downloaded and the absence of the original files will not pose a problem.