"Ancient History" is sort of a sidenote to the main story of Custer and Co. as this book (the fourth in the series) contains back story for the Saint of Killers, Arseface, and Jody and T.C.
The best of the bunch is the Saint of Killers' story. From late 19th century wild west, cowboys and injuns, to the gates of Hell and beyond, we see how the Saint became the cold blooded killing machine he is in the main story. Carlos Eszquerra supplies some fantastic drawings for his part in the story, the Hell sequence. His Devil is brilliant and you find out his first name too. The story takes up a full half of the book and is perfect reading for upcoming Hallowe'en celebrations. That is, read it by candlelight with a drink and ignore the brats at the door.
Arseface's backstory isn't that great. If you've read the first book (and why haven't you?) you'll know he looks the way he does because of his own actions and some other details. It's elaborated on more here but doesn't really provide that compelling a story.
Jody and T.C.'s story is a satire on action films and has a fun story of the pair taking apart an international mobster's hitforce in the swamps of the South. Some really creative deaths in this part of the book.
It's a great addition to the brilliant story of "Preacher", fleshing out the supporting players for the reader and providing some excellent reading for a couple of hours.