Fort Strong is a small military city located on a peninsula occupied by supermutants. You will get rid of them during one of the first quests done for the Brotherhood of Steel.
On the station north from Fort Strong you will find a power armor.
The most important building in Fort Strong is armory. You will meet another dozen of supermutants here.
In the southern part of the Armory's ground floor you can find U.S. Covert Operations Manual. In the same room, in the corner, you will find a master level safe. Inside you will mostly find ammunition.
If you have active quest of searching for technical documentation for the Brotherhood of Steel, you can find many of them here.
Don't forget to use the elevator and move to the research facility underground. There you will find other supermutants and large number of valuable items.