Taking it one step further, don't forget about other units in the same campaigns, both north and south. If you find the unit you are researching was in a campaign, skirmish, battle, etc, you can usually identify nothern units opposite the southern troops of your research (or vice versa if you had yankee ancestors). Reading the opponent's history may provide you valuable information to add to your unit's history.
Occassionally I find a book with information concerning my area of research which haunts me until I buy it. If others have this problem, I suggest going to the ALIBIS.com website. They sell new and used books through hundreds of book dealers. The price ranges are from cheap to more expensive than I want to pay, but many old and/or rare books have been reprinted and made available at reasonable prices.
Another benifit of using Alibis is you can find and purchase the ORs individually. Since the Trans-Mississippi Department is my main focus of research, I have only purchased those ORs relating to that theater of the war.