You're guildmaster of the Thieves' guild, and you need reference for each shadowmark? If Gallus was still alive, he'd boot you out of the cistern faster than you could say "larceny". Still, I think it'd be easier to make a list of shadowmarks and meanings, and add them to the front page of the book. You'll still have to refer to it, but it'll be easier than flipping through the pages. After all, where's the fun in having everything spoonfed to you?