There was a book or series of books I used to read when I was younger, probably late 90's. I think they had a fantasy theme. All the way through the book there were little riddles and puzzles to solve before you had to carry on reading. For example it might have got to a page about a door and a lock, and then you had to complete the riddle to find the number that opened the lock? I remember them being quite intricate and I loved them but it's such a loose memory for me I can't place a name to it. Does it ring a bell for anyone?