I've looked around for an answer to my question, but have only succeeded in confusing myself.
I've been changing the font for my ePub files to Fontin with Calibre using Zelda Pinwheel's excellent instructions in this post:
This has always worked perfectly for me with Calibre 5x, but since upgrading Calibre to 6 it doesn't seem to work.
I'm inserting the font information into the "Extra CSS" dialog box under the "Look and Feel" option exactly as I had done in Calibre 5x, but the ePub produced does not use the Fontin font I requested.
I'm sure I'm missing something simple, and would appreciate any pointers on how to make this work!