If you do not wish to spoil the difficulty in finding the dungeon please do not continue reading. This is for those people who wish to skip straight to the dungeon.
If you wish to head straight to the dungeon, it is located on the large icebergs north of Dawnstar. Alternatively, you can use console and type "coc sunkencaveexterior"
If you wish to find the journals for roleplaying but do not wish to waste time, here are the locations:
1. A satchel with 2 journals is found on the stretch of road between whistling mine and winterhold, sitting on a rock.
2. The second satchel is located in Whiterun hold, located next to the river, slightly north of the portion of land that juts out into a spit (this has a chest and usually a wild animal encounter), next to a burnt corpse.
Keep in mind that if you don't want the journals, they are not required to find the cave, and you can go straight there.
Once you've reached the treasure room of the Sunken Cave dungeon, you will find the items you need in the chests, (recipe tome and amulet of power) which you can take, and forge the amulet of power (i), and then the ring itself in the skyforge in Whiterun. The tome of Rescript should have more info about this.