From some cursory reading, silverfish like starch and paper.
I found only one in a room I don’t use much, and sometimes as a study.
Do most insects come from the garden, or is one just carried in, from groceries, etc?
I couldn’t get a picture since it was too quick for me.
How would I get rid of this, aside from cleaning a bit more?
Sorry to hear about your problem.
Silverfish are fast nocturnal creatures which can reproduce rapidly in ideal conditions. They thrive in damp and dark environments, particularly if they are not being disturbed often. Although they do not cause any harm to humans, they do destroy books, wallpaper and clothing.
Your search into their meal preference is correct. They also like sugars, cellulose, shampoos, glue in books, as well as, linen and silk.
As to how it got in the house, it is difficult for me to tell you with certainty. It could have come in a box or old books or magazines; if the room is in the basement, it could have come through a crack in the windows or wall.
In terms of control, I would suggest ensuring the room is dry and clean; eliminate any damp boxes or fabrics; vacuum often to eliminate their eggs. Check that baseboards are sealed, particularly if the room is in a basement. Some people claim that placing cinnamon and ground cloves repel these insects, however these do not control an existing problem (If you see centipedes, earwigs and spiders, you may want to leave them alone as they are known to feed on silverfish).
Please find the attached information from the Health Canada website on this topic and some recommendations on dealing with this pest: https://healthycanadians.gc.ca/environment-environnement/pesticides/silver-argent-eng.php
Hope you find this information helpful.