Catton's books are easy to read and provide a good overview of the subject in question. If that's what you're looking for, then you'll be happy with any of his books. If you want in-depth history then you need to look to other authors.
Just as an after thought, "A Stillness at Appomattox" is the third book of Catton's Army of the Potomac trilogy. You may want to read the other two first, if you haven't already done so.