The fort is not open to the public and attempting to enter will cause its inhabitants to turn hostile. Thorald Gray-Mane is being held captive here and can be found in the dungeons. You will be tasked with freeing him during the Missing in Action quest. There are also other non-important prisoners in the keep, but when freed, the player will be unable to communicate with them, as they simply order the player not to speak with them.
The skill book, The Legendary Sancre Tor, is found in the captain’s room, which upon reading raises two-handed weapon skill by one point.
The book, 2920, Morning Star, v1, is also found here, in the dining area behind the bar, which upon reading raises one-handed weapon skill by one point.