In my humble opinion, the most important rule to follow in mystery/suspense writing is to give the reader a sporting chance! I've written plenty of novels, and the most successful ones have been those where the reader had a few plausible guesses about "who done it!" Personally, I rarely guess the ending of a mystery/suspense novel and if I do, I’m kind of disappointed. I want that author to stump me with a clue I didn’t notice or a twist I couldn’t see the significance of at the time. It’s a game I don’t want to win, but I do want the author to play by the rules and that means, sprinkling in those clues along the way. Bury them in backstory, dribble them in dialogue, subvert them in setting, but readers need to have the chance to solve the mystery themselves. The key is to sprinkle, not dump! The great Agatha Christie said she was “always wary of putting too many false clues into the plot, because with so many things to unravel the book would be not only difficult to solve but also difficult to read.” And there must be herrings, red ones! Just as the fish could be dragged across the trail to confuse the hounds, a red herring is a false clue dropped into the story to confuse the reader and perhaps the protagonist as well.
Do you usually solve the mystery before the end of the book? Does that leave you satisfied or disappointed?