Reddit mentions of The X-Files: The Official Collection Volume 3 - Conspiracy Theory - The Truth, Secrets & Lies

It was! Great content with plenty of behind the scenes info, a lot of work on the looks/aesthetics of the thing.

If you're interested, Titan reprinted a selection of their articles into 3 volumes a couple of years ago. You can "look inside" a bit with Amazon previews:

https://www.amazon.com/X-Files-Official-Collection-Agents-Syndicate/dp/1782763716/

https://www.amazon.com/X-Files-Official-Collection-Monsters-Villains/dp/1782763724/

https://www.amazon.com/X-Files-Official-Collection-Conspiracy-Secrets/dp/1782763732/

I think The Complete X-Files: Behind the Scenes, the Myths and the Movies is a book which covers this sort of stuff. I haven't read it yet, because I am waiting for the Revised and Updated Edition which includes Season 10 to come out in November.

There is also The X-Files Official Collection, which is content from the official X-Files magazine, which was released while the original series ran, collected into three volumes:

• Volume 1: The Agents, the Bureau and the Syndicate
  
• Volume 2: Little Green Men: Monsters and Villains
  
• Volume 3: Conspiracy Theory - The Truth, the Secrets and the Lies