The Killer Puzzles series was authored by Kjartan Poskitt, and spanned four titles released in the mid-to-late nineties. They diverged from the Usborne Puzzle Adventures and other series of around the same time in several aspects: apart from a rather twisted sense of humour, they were also extremely difficult, to the point that Poskitt received almost as much correspondence asking for help from adult readers as he did from children - because the series also did not include any answers or solutions.
Killer Puzzles also had one other selling point: if you managed to complete every puzzle in the book, then you could solve its big secret. In three of the four entries (to wit: Decode the Deadliest Joke in the Universe, Titus O'Skinty's Gruesome Gameshow and Attack of the Killer Puzzles), this was a coded message of some kind, although the specifics as to how you decoded it and how the puzzles gave you the key were very different in each book. The other book, Find the Phantom of Ghastly Castle, took a totally different approach: for one thing, it wasn't just a puzzle book, it was a sort of adventure gamebook.
And "sort of" has always been good enough for this blog.
