Just finished reading Desert Oath and it was a pretty good read. You really get a feeling for the relationship between Bayek and Aya, as well as where theor difference in ideologies are going to lead to later in the game. Something new however is that unlike previous Bowden books, the story is still a prequel, but actually stops before we get into the game. Traditionally he writes the first half as new story and the second half as a retelling of the game, but this time it's actually separate.
You have a little over two weeks to read it before the game comes out and I highly suggest picking it up now. It's a pretty quick read and really worth it for growing these characters further