Grant Wins the War: Decision at Vicksburg ($13.17 US), by James R. Arnold, is free for UK customers in the Kindle store (UK link). This one is from publisher Wiley and I haven't been seeing their books drop to zero for US customers.
Book Description
Vicksburg is the key. . . . Let us get Vicksburg, and all that country is ours.--President Abraham Lincoln, 1862
In a brilliantly constructed and powerfully rendered new account, James R. Arnold offers a penetrating analysis of Grant's strategies and actions leading to the Union victory at Vicksburg. Approaching these epic events from a unique and well-rounded perspective, and based on careful research, Grant Wins the War is fascinating reading for all Civil War and military history buffs.