Edited on January 23rd, 2009 to add links to the books in the Adept series.
The blurb on the back of the book:
What magic can stop Adolf Hitler - History's most evil Black Magician?
The year is 1940
Hitler's Germany is about to employ the secret arts of evil witchcraft to destroy England. What can stop them?
It is the mission of John Graham, colonel in British Intelligence, to stop the onslaught of evil with an extraordinary strategy that defies all the rules of twentieth-century warfare: Unite the different witches' covens throughout England, drawing upon powers that reach back through dark centuries, in a ritual of awesome sacrifice on the first night of August, the magical Lammas Night.
Lammas Night is an older book. I tend to say it's set in the same world as the Adept series: The Adept (which I'm reading now), The Adept: Lodge of the Lynx, The Adept: The Templar Treasure, Dagger Magic and Death of an Adept. Thing is, it was written before the other series, so properly, I should say they were set in the world of Lammas Night. However. the books don't overlap until The Templar Treasure, when Gray (John Graham) plays a role. There's also, if my memory is correct a short story in one of the Templar anthologies that Katherine Kurtz has edited (Tales of the Knights Templar, More Tales of the Knights Templar and Crusade of Fire) where Grey and Adam are interacting.
Good luck finding Lammas Night, although it's well worth the read. I ended up searching used bookstores for a few years, but it was well worth it. I've read the book before, but this last time the ending brought tears to my eyes.
Katherine Kurtz uses foreshadowing very well in this book and the others that I've read. As the book builds I found myself hoping that this time things would work out, even though I've read the book before. Of course the story didn't change though. Every time I wish it would. I don't want to say more for fear of spoiling the story, however.