Squid Ink is a magazine that Squidward reads in the episode season 4 epsiode ,Bummer Vacation. It is probably an art magazine like the book Art. You can seeSquidward read it where SpongeBob wanted to eat a Krabby Patty with Patrick in the Krusty Krab. It actually looks more like a comic than a magazine.
This magazine has a yellow cover. And on it says "Squid Ink" in blue writing, and under that there is a spot of ink and on it is pen.