So I recently found in the Thieves Guild quarters a book called "Shadowmarks." After opening and reading it I found out that it talked about marks that the Thieves guild use across Skyrim to inform fellow Thieves about various things. I was wondering if anyone has actually found any of them? also they do have symbols for danger, escape, and loot so they should be useful for rouge and thief characters.

Yes, they are hidden along the routes you take throughout the TG quest line. One that comes to mind is near the balcony in Solitude

there everywhere. on many houses in the cities and shops and so on. I remember two in Whiterun, for example: Jorrvaskr, House of the battle-born. I know there is one at the smithy in Solitude also. Like I said, they are everywhere, just keep a lookout for them on wooden pillars or the likes near entrances to buildings.

Those shadowmarks are confusing as all hell.

if you look clouselly to most home and shops facades they are allmoust all marked with them, mostelly on the left down corner facing the doorways

Ah cool, I know I don't play a stealth type but that shall still be useful