Harry W. Pfanz, Gettysburg - The Second Day. Comprehensive and highly detailed for both sides. Specifically covers the southern half of the battle, from Cemetery Ridge to Big Round Top. Several maps showing regimental and battery positions at specific times. Individual unit accounts may be better understood if you have Pfanz close at hand to compare and verify.

http://www.amazon.com/Gettysburg-Second-Day-Harry-Pfanz/dp/0807847305