Skyrim Secondaries- Forbidden Legends: The Mage of Solitude

Near Solitude, you'll be able to find the tomb referenced in the book- Folgunthur.  Head down from Solitude to the rivershore, and then cross, heading for the small mountain that supports the Blue Palace and the far end of the bridge that makes up half of the city.  There you'll find a small smuggler's camp with a book that improves Pickpocket, a small amount of food, and a cooking spit.  That camp should be close enough to Folgunthur that its icon will appear on your navigation bar at the top of the screen- follow that to reach the ruin, being cautious for wild animals and bandits as you go.

There is another camp outside of Folgunthur, composed of four tents and a large fire.  This camp, and its own small stores of food, have also been abandoned, though you can find a book here- Daynas Valen's journal.  As it turns out, Daynas Valen was an elven mage of some sort who previously dug about in Folgunthur, in search of a mystic amulet.

Coming out empty-handed, he went on a research trip first into Solitude and then outside of Skyrim itself.  On his return, he was able to talk a somewhat unscrupulous adventuring group into coming into the ruins with him.  Before he entered, though, he burned all of his research and notes to ensure that nobody else could get ahold of the amulet he coveted, regardless of whether or not he would obtain it himself.

This moves your quest marker into Folgunthur itself, so make ready for the large dungeon.  Empty out your inventory into your house's storage, prepare whatever follower and/or dog you intend to bring with you, and make sure you do a quick scan over the area just outside Folgunthur's front door, as there's a burial urn and a soul gem sitting around out here.

Once you're ready, it's time to, as the quest marker declares, find out the truth from Daynas Valen.

On entering Folgunthur, you're confronted with a hallway that descends gradually through a series of stairways- but is interrupted in the middle by a quarted of corpses around two apparently ornamental dragon heads.  Take care, though- these are actually a trap, triggered by a floorplate just before them.  Circumvent the spears by carefully avoiding the trap's trigger, then head down the next stairs- you'll find a spot that a precious claw should be used at- but the gate here is open, a porticullis that would bar your way to the next room.