The Clown Service

The Clown Service is a speculative fiction/spy novel by Guy Adams. The main character, Toby, is a British spy with PTSD who gets transferred to Section 37, a neglected branch of the British intelligence operations dedicated to dealing with the paranormal. In his new job he finds out that there is a lot more to the world than he ever thought possible. Soon Toby has to step forward and save the country from a peril he would have found ludicrous mere days previously.

It is a very Troperrific novel.

Tropes used in The Clown Service include:

Addictive Magic: Or more accurately, addictive astral projection.
Astral Projection
Complete Monster: Olag Krishnin. The book even makes it clear that he was evil from childhood.
Dark World: There is an alternate version of the world that can be accessed by certain people with special powers but who can also take other people with them. It is inhabited by monstrous, distorted versions of people and animals.
Demonic Possession: Or possession by whatever the Legion is.
Disability Superpower: Who knew that PTSD could be turned into a weapon against extradimensional horrors?
Fantasy Kitchen Sink: Pixies, leprechauns, yetis, Chinese dragon gods... The list goes on. It is implied that the list goes on very long indeed.
For the Evulz: Krishnin doesn't care about serving the Soviet Union. He only does what he does because he can.
Functional Magic
Golem: Made with the traditional Jewish lore, with the true name of God and all.
Government Agency of Fiction: Section 37, also known as the Clown Service.
Year Inside, Hour Outside: Time works strangely in the alternate version of the world. Five minutes can be an hour in the normal world, but it varies.
Intangible Man
Living Statue: A bust of (Herbert?) Kitchener.
Our Zombies Are Different: Type C, like golems but made from dead people.
Perception Filter: Cyril is permanently affected by one. He has to remind his wife every morning about his existence, but the condition makes him an excellent infiltrator.
- A warehouse is hidden with a spell to make people not notice the building even when they're walking straight past it.
Race Against the Clock: Subverted. The number station countdown is nowhere close to zero when Krishnin puts his plan into action.
Rogue Agent: Krishnin.
The Mentor: August Shining to Toby.
Time Machine: It works as a portal that cannot be fully controlled and can only function for short periods of time and is also very dangerous to use. The inventor wanted to file a patent.
Who Wants to Live Forever?: Krishnin never considered the downsides to being unkillable.
Zombie Apocalypse: Krishnin's plan unleashes this on England.