Containing Bitter Seeds, The Coldest War, and Necessary Evil, the Milkweed Tryptych is, frankly, one of the best collections of stories I've ever read. I devoured these books. I read the first two over the span of about three hours, and finished the second one a few days later after I'd done mundane things like going to class and doing assignments.
Bitter Seeds introduces the reader into an alternate World War 2, in which the Germans have found a way, using human experiments and battery packs attached to the brain, to create super humans. In order to fight this new German threat, the British must consult usually-hidden warlocks, who can converse with the ancient Cthulhu-like Eidolons. The warlocks can buy the help of the Eidolons, but the price continues to get higher and higher.
I'm going to jump right into appeal now because this is the only book in this project where one story element completely overshadows the others. Story is everything in this triptych - as interesting as the characters (10%) are (including the terrifying clairvoyant Gretel and naive warlock Will Beauclerk), and as engaging as I find alternate history settings (10%), the sheer genius of the story (80%) eclipses it all.
I am very careful to not classify this as a series, because it isn't. The stories are not linear works, though you do need to read them in order. The first book is good. I kept flipping flipping the page on my tablet faster and faster, and started the second book as soon as I'd closed the first.
But it's in The Coldest War where Tregillis' genius shines. As you read through, you realize the scope and terrible vastness of Gretel's powers, and the second book culminates in a twist that had me actually sitting up in bed and swearing out loud. It is one of the most visceral reactions I've ever had while reading a book. I frantically reopened Bitter Seeds and reread certain parts frantically. Everything you thought you knew about what you'd read in the first book is completely overturned by the end of the second, and it makes you want to go back and reread the first one. (Which you kind of do in the third book.)
I don't want to say any more, as the experience of reading these three books is a joy I don't want to spoil for anyone. I will say that Tregillis' craftsmanship of his narrative is breathtaking.