Collect all the brown bonus books during a mission to complete the photograph at the end and unlock a bonus mission. Note: The "Saving Private Smith" mission does not have an associated bonus mission.
Always be aware of your enemies vision range. Do not assume that they cannot see you. There is a good chance that the soldier will move around and will spot you. Use the "Eye" tool and place it on an enemy soldier to reveal his vision range. Remember, if your commando is standing up and is in the "light" area of the vision range, he will be seen but it will take about five to ten seconds for your enemy to react. During this time, go into crawl mode. If you are in crawl mode, and are in the "light" area of the soldier's vision range, you will not be seen. However, if you are not using the spy and are wearing an enemy uniform, you will not be seen even when standing. However, the darker area of the vision range is the hot zone. You will be seen, and the enemy will react automatically no matter what, except if you are using the spy.
Always try a stealthy approach rather then using guns. If you are using the Green Beret, you can sneak up on the enemy and cut him. Remember to pick up his items. This will not alert other soldiers unless the "sound bubble" appears, which is rare. A good tip for handling many soldiers with only a gun is to use the guard vision. Guard vision will place your shooting range in front of you. If an enemy soldier goes onto that vision range, the enemy will be killed as fast as possible. This is useful method and it does not not matter how skilled you are at shooting in the game. The CPU will always do better. Additionally, use the following trick when you have the diver on a mission with an enemy that you get behind to throw the knife at. The game's line of sight feature works as follows. When you lay in the light area you will not be spotted, but if you stand up a blue sight will appear which indicates your enemy does not know who you are. The longer you stand up, the more suspicious he will become. However, if you lay down in the darker area the same thing will happen. Use the diver then take out his knife. Next, go up to the enemy then lay down in the darker area and crawl towards him. The blue sight will appear, but do not worry as he will not notice you for awhile. Once you are in range, throw the knife. If done correctly, you will kill him while remaining safe.