Looking at the item it appears to me to be some type of book. I doubt it is a day book or diary of any sort but it could be a soldier's pocket Bible. If Captain Stuart was anything at all like Gen. Stuart I could easily see him posing with his Bible.
Now, that's a wild guess but before we dismiss the thought let's look at a pocket Bible dated 1862 and see if there is any similarities. Check out this page...
http://www.rubylane.com/shops/charmingandcheap/item/8009