Speak to Frelia at the weapon shack outside the Bannered Mare and ask about her son. After that, go with her to her house and speak to Avulstein . Now we need to go get proof, so leave the house and enter the Battle-Born’s house (Save before doing so, and remember to be sneaky)!
You can do this in broad daylight and get away with it, (although there ARE people inside the house so stay in stealth mode!). Enter the room to the left and lockpick the expert door and read the book (take it, of course). Note that you can also find the TWO HANDED skillbook "Battle of Sancre Tor" inside the house as well (it’s on the book case in the left room) . Return to Frelia and Avulstein and give them the missive you found.
Now, you can either join Avulstein and go kill some Thalmor, go reason with them (which won’t work), or go kill them on your own. Choose what you will, (I recommend joining him). Now; head to Northwatch Keep . It is located at the very northwest corner of Skyrim, so hopefully you have the map fleshed out quite a ways (and yes, you can fast travel there).
If you choose to join Avulstein, you will have to go east down the road a ways and talk to him, then storm the keep. Either way they won’t let you in, so kill them all (two archers and about 3-5 soldiers). THIS is why having Avulstein and his friends is handy! Once you’ve slaughtered the outside soldiers, head inside the keep.
In the second room, you’ll find a soldier and a mage. Kill them both. Follow the path and as you descend, you’ll find an archer and two more soldiers. B e sure to check out the bar carefully for the ONE HANDED skillbook "2920, Morning Star, V1" .
In the next large room, there are three soldiers and a mage. Go through the lower door here before going upstairs and loot the room. There’s a TWO HANDED skillbook here named "Battle of Sancre Tor" , which is the same book you could have found earlier in this same mission, so you should REALLY have it now.
Go up the stairs and in the next room, you’ll find a soldier looking away from you. Take care of her and go through the opening to find a mage and Thorald . Rescue him and then it’s time to leave this place. Head through the prison section (where you may find two more guards) to quickly leave the fort.
Note that the door up here requires the fort key, which you should have if you’ve been looting bodies as you’ve been killing (which you should ALWAYS do). If you don’t, you’re going to have to go back the way you came.
Once you’re outside, talk to Thorald and he thanks you for saving him and asks you to go speak to Frelia. Go to her and repeat what Thorald told you and you’ll receive a Steel Greatsword of Scorching . This quest will also now be over.