Lindsley's Annals contains a detailed regimental history of the 8th, written by General Dibrell. Cornell University's on-line library has it on their web site. You can download and save it on CD, I did. It's a great reference for anyone researching Confederate ancestors from Tennessee. I'm not sure how the 13th became the 8th, but there were a lot of quirks in the way Confederate units were designated. I am currently doing research on Wheelers last raid into Tennessee in 1864. The 8th TN was part of this raid. They had a very colorful career as a unit. On August 10th, 1864, when Wheeler began the raid, General Dibrell states that "the Eighth Tennessee, reduced from nine hundred and twenty-one men when mustered into service in September, 1862, to one hundred and forty effective men mounted and ready for duty, marched with the brigade and Gen. Wheeler on his contemplated raid into Middle Tennessee." You can see that they had suffered greatly during the war and especially during the Atlanta Campaign.

Good luck with your research
Paul Maloney