Byron aside, I recommend Tim Powers' novel "Declare" -- enjoyed that one more than "The Anubis Gates". Keywords: fiction, espionage, thriller, 40s-60s ww2 / cold war, occult horror, historical fiction.

For anyone who enjoys the mix of espionage & occult horror themes in "Declare", there's a couple of similar themes, with differences, in Charles Stross' Laundry Files series. Stross' series features a lot more government & IT bureaucratic comedy/horror.

I'm not sure there's a direct Byronic connection to the Laundry Files, but one of Stross' axioms for the Laundry Files universe is that magic is a branch of applied mathematics -- understanding the right theorem or having a person (or circuit) execute a certain algorithm has occult consequences. So we can at least indirectly make a connection back to Byron via Ada Lovelace.

https://www.goodreads.com/book/show/190554.Declare

https://www.antipope.org/charlie/blog-static/2020/10/the-lau...

I've been wanting another book like Declare ever since I read it - annoying to find your absolute favourite genre appears to consist of exactly one book. Not that the Laundry isn't fun and good :) but there's something about how mystical Declare is, like a Crying of Lot 49 or Foucault's Pendulum but that actually wants to get somewhere with its hints of a bigger world rather than just tease the reader with them.

Depends on what you want from a book "like Declare". While it isn't about spies, Powers' "Last Call" is basically "Declare" only about the early mob-run history of Las Vegas as the background instead of the Cold War.

“Last Call” is amazing. I’ve read it 3 times, and I swear, the last 150 pages are still some of the most intense I’ve encountered.

I've read most of Powers' other books and liked them, but none hit quite the same. Declare had a gritty majesty to it with the geopolitics, the others had a bit too much of the fantastic about them. Too obviously stories to fit the history, rather than... history turned into a story, I guess? The spy angle really lent itself to his writing, for me.

O fish, are you constant to the old covenant?

"The War in the Dark" by Nick Setchfield scratched a similar itch to "Declare" for me. (There's a sequel too, but I've not read it).