This building is a huge edifice that houses library, and that has been inhabited by super mutants. Under the library, there is a metro station that is connected to the building -- Copley Station. In the building, there is an abundance of valuable items.
You can get inside through the metro station, or through the entrance in the Western wall of the building. In both cases, you are going to need advanced level of Lockpicking . If the level of your Charisma is sufficiently high, during the conversation over the intercom, you can convince the managing system that you are a worker. This eliminates the necessity to break in and robots inside will not attack you.
In the building, there are a dozen-or-so robots that protect it. Not only do they help you fight mutants, but they can also be looted for valuable crafting materials, after they are destroyed.
In the North-Western part of the building, there is a room, where you find Intelligence Bobblehead. It is on the computer's console. In the corner of the room, there is a stash locked with master lock. Inside, there is plenty of ammo, food, meds and Stealth-Boy.
If you have the quest to search for technical documentation for the Brotherhood of Steel active, you should give desks and cupboards a thorough search, because there are quite a few of them here.
In the South-Western corner of the building, there is a book dispenser. For 50 tokens (you receive them for returning overdue books) you can receive an issue of Massachusetts Surgical Journal. In the library, there are not enough books to receive 50 tokens. You also need to search for them in other locations of Commonwealth. Search especially university buildings and schools.
On the metro station, under the library, you encounter another group of super mutants. It is a good idea to ransack the entire underground, because you find valuable items here. You should especially pay attention to the Northern part of the station, where you can get across one of the metro cars. On the corpses of merchants, you find around 100 caps.