One good primary source is Joseph C. Snider's Journal of the Civil War, 1861-1864 -- Snider was born on 10 March 1839 in Harrison County, Virginia and served in Company C of the 31st Virginia. A diary exists from Sgt. Squire Newton Bosworth in Company F, and a published diary of James E. Hall of Company H may also cover the late 1862 period, but I have not saved information from these accounts concerning Fredericksburg. Check also the www.militaryhistoryonline.com/geneology website for tidbits on soldiers from the 31st.