You always knew there was something more to the history of the Second World War, didn't you? After all, that's why Western civilization has been glorifying that Epic Win in film, literature, and gaming culture for decades: we've wanted to know the stories underneath the grim figures, and to find ways to humanize (should that be possible) such an inhuman struggle.
Although, it might be better said that you should aspire to a skein of plausibility, since... I have no idea how to infuse your alt-history with that all important element.
So ,without further ado:
1. Plausibility: You might ask, "how could Nazi supermen and British warlocks, embroiled in the greatest war ever fought, possibly be plausible?" Just... read the book. The fact that Tregillis orchestrates "Bitter Seeds" so that it seems like, just maybe, this might have happened and, due to various state secrecy acts, we never heard about it, makes it all the more satisfying.
What's satisfying? How about:
2. Trauma-induced supermen: You always knew that being picked last in gym class would result in super powers. I mean, I knew that, at least. Bitter Seeds is here to show you just how much worse your childhood could have been - and how much more awesome your adulthood would be if that was the case!
3. Wizards that are way cooler than Harry: There's no Hogwarts in Bitter Seeds, just self-mutilation and a battle of wits with supernatural entities. Yes!
4. Nazis: I'm pretty sure that humanity will never get another enemy as perfect as the Nazis, and, thusly, we should never stop villifying them in our culture. This isn't exactly an original idea, but if you want your heroes to have universal crowd appeal (minus a few douchebags), pit them against Nazis. I wonder if Hitler expected his movement would live on forever... under the crosshairs of my Wolfenstein 3D et al?
At this point, you've probably realized that there's at least a few elements Tregillis uses in Bitter Seeds that I'd love to inject into my own stories. Of course, the real brilliance of Bitter Seeds isn't that we've got supermen and warlocks; it's that he injects these classic fantasy tropes into his alternate history, and does it seamlessly.
The greatest thing? Tregillis is a brilliant writer. I literally don't hear the pages turning - that's how good a writer Tregillis is. I've always found that books I read quickly are better than the books that take longer - not because I don't enjoy dwelling in made-up universes, but because the pace at which I read is an obvious indication of just how much fun that universe is.
And, yeah... I read Bitter Seeds pretty quickly. You probably will, too!