Firearms are unlocked through French missions and are a pain. Overall its a series of 3 different missions in three different battles with 10+ star difficulty, in one you find wood for the stock, another is sulfur, and the last is iron for the barrel. After all 3 of these French missions the librarian gives you the firearms book. Build up your reputation with the French because the librarian is your only way of getting the missions for the components.
After you beat the story line quests, you will see the two men talking about strategy when you check gossip. To get the Engineering Tome, you need to 100% research Sword/Horse/Bow so that an Avatar of war will show up and give you the Silver weapons for that squad. You also need to have 70% learned on ALL other Books as well. Once you have 100% on the mentioned 3 and 70% on all the others (Firearms included) and the Avatar will reappear and give you this tome. The Avatar shows up in the Tavern.
Good luck and good gaming.