I had also read that the first book was related to Riptide, and it was not, which is disappointing, and the first book wasn't even about anything nuclear, that was the second one.
This first book sets up Gideon. He is an art thief who steals for the thrill (and to, in his estimation, save works of art from moldering in the back room of a museum). When he discovers his father, supposedly killed as a terrorist, was framed and murdered, he gets into a nuclear program to avenge his father.
That's not what the book is actually about.
What it is about is Eli "I will never learn my lesson" Glinn hires Gideon to do a job for EES.
(Completely random fact, I applied to be a secretary at a place that had "engineering solutions" in its name. I was not hired. I assume Glinn tutted and threw my application into a fire.)
Gideon must find a Chinese scientist coming to America and find out what secret he is bringing with him. Nothing goes according to plan, at all. Gideon meets a prostitute who falls in love with him within the space of a day. He also meets a sexy CIA agent. He is mostly oblivious to the assassin on his tail almost all of the time, and it's actually kind of embarrassing for him.
However, things work out.
That's...really all there is. It's mainly Gideon sort of stumbling around and accidentally doing things right. Later, Glinn points out that Gideon is an intuitive genius, although I think that's going a little far. I would say he's just a lucky bastard.
Gideon's Corpse starts out right after the first one. Glinn sends Gideon to visit an old friend of his from his nuclear work place. The man is crazy and insisting that he has been experimented on by not aliens, and is holding his landlord hostage. When it is discovered that the man converted to Islam and is extremely radioactive, chaos breaks out. Gideon liaises with an FBI agent who isn't Pendergast, and they investigate his friend's past. Refreshingly, they consider the possibility that the man's imam is a terrorist, and it isn't framed as prejudiced nonsense. They discover it isn't him through the process of elimination, not with an anvilicious message that believing Islamic terrorists are a thing in a world where Islamic terrorists actually exist is wrong.
Gideon meets his old friend's favorite writer, and the writer's Obligatory Hot Daughter. Sexual tension ensues. However, Gideon gets too close to the truth, and winds up being framed as a terrorist himself, in the most egregiously dumb way possible. It still takes his FBI friend a while to figure out that the whole thing is nonsense. Gideon takes the Hot Daughter hostage, and they fall in love. Because of course they do. Gideon later discovers that the writer was behind it all along, and it's a fake nuclear threat designed to steal the last small pox and unleash it on China, because...um...
Gideon must kill the writer, and Hot Daughter no longer wants him.
But there's no time for sadness, because...
The Lost Island isn't in fact lost, and WHOA NELLY do things get weird. Spoiler alert, Gideon has an AVM, which is a mass of arteries that get stuck together in your brain and will one day implode and there's no symptoms so you won't even know it unless you've had an MRI and one day you will just die. And there's no cure.
SLEEP WELL TONIGHT KIDS.
However, Glinn thinks he's found a cure-all. It isn't an arcanum, we're saving that plot point for DiogeVader. Gideon must steal the Chi Rho page from the Book of Kells, and we get some back story about St. Columba, though nothing about his infamous Druid Trolling. Then Glinn destroys the paint on the Chi Rho page.
My face looked something like this.
After a bout with some crazy treasure hunters, the pair wash up on an island of FRICKING LOTUS EATERS. Because the lotus? That's the cure all. They worship a Cyclops. (By the way, the Chi Rho page was made of Cyclops skin.)
The Cyclops is real. When Amy mentions Polyphemus, the Cyclops recognizes the name.
LOOK YOU REALLY CAN'T GET MUCH WEIRDER THAN THIS, so let's just say some King Kong nonsense happens, Glinn screws up even harder than he did before, and it ends with everything going to hell in a handbasket.
For the record, the lotus actually is a cure all. Except for Gideon, because he needs all the angst.
But none of that matters, because we have to kill Audrey II.
But I've been spoiled by Pendergast. Is it October yet?