When I was younger, I read a lot of Encyclopedia Brown. In case you don’t know, those books are structured uniquely: each book contains five or six stories about Encyclopedia Brown investigating some type of mystery. At the end of each story, Encyclopedia Brown would have solved the mystery, but the question was put to readers as well— they could figure it out for themselves before finding the real answer in the back of the book. It was like a book of narrative riddles, meant to cultivate a group of miniature Poirots and Marples.
I never waited to figure out the answer myself. I always flipped to the back of the book immediately and read the answer. I enjoyed wondering, then being satisfied, more than I did solving the mystery myself. After all, if the author had already figured out the answer, why waste time on it? I was perfectly happy to let someone else do the work, as long as they got it right.
However, when I was forced one way or another to try and figure out the solution myself, I never enjoyed it. The work was already done— why did I have to repeat it? Furthermore, I read Encyclopedia Brown so I could enjoy his superior brain power, not so I could (gasp!) learn something. I wanted to see how smart he was and enjoy that. Once I was forced to unravel the mystery myself, however, it became less enjoyable and more difficult— work, not play.
To a certain point, readers feel the same way about mysteries in other areas of fiction. The mystery is there to be solved, but by the main character, not by the reader; the same way that the epic battle is there to be fought by the main character, not by the reader. The reader has no input in the story, no say in what happens. Thus, if the readers solving the mystery won’t help, why do it? It’s just extra work.
This is why the main character usually solves that mystery. The reader enjoys seeing the puzzle pieces fit together and revels in the main character’s smarts, but never takes active part in solving the mystery. The reader is the passenger, not the driver.
However, the reader might still try to figure out the answer, or stumble upon it accidentally over the course of the story. Brandon Sanderson often says he wants the reader to unravel the mystery a paragraph before the character does. I remember wanting to shout at the Boxcar children for seeing so little (when I was younger, mind you) when I figured out the mystery a chapter earlier than they did. Over the course of a good mystery, the reader will have the same information that the main character does. They have the opportunity to solve the mystery before the character does, but they can’t enact its solution. What happens when these secrets come to light? How will the main character choose to reveal his information? The reader doesn’t know, so they have to read on.
But if the author forces the reader to solve the mystery themselves, instead of merely allowing it— if the author tries to get the reader to figure it out without flipping to the back, does all the fun dry up? Does all that satisfaction of a mystery unraveled turn sour?
I think it depends. If your main character is a Miss Marple and solves the mystery without telling the reader what happened, and acts accordingly without any sort of reveal, that will be disappointing. That’s Sherlock Holmes saying, “Of course he killed her” without explaining himself. It’s just not fun. At that point, Watson needs to scratch his head and represent the reader, getting an explanation out of Sherlock before things get too far. An unexplained mystery is a broken promise, in that case.
However, there are other circumstances. Say your main character is not the detective. The mystery exists, but its solution is not instrumental to the plot. Its solution would help the reader understand the plot, but the character might already know, or doesn’t care. I think in that case, very carefully, the author could pull it off; the author could present all the clues for the mystery and leave it to the readers to solve for themselves. Essentially, the author must not promise that the mystery will be solved by the end— they must not solve it in the back of the book, then force the readers to do extra work to obtain the same answer. They present the story, with clues to the mystery, but leave the solution as an added bonus for astute readers; much like Brandon Sanderson’s Cosmere. The trick is not to promise it.
Here’s an easy test: does your plot or character development hinge on this mystery? Imagine a tough main character and her sidekick trekking across the wilderness. The sidekick has a problem with pineapple arising from his past— whenever it’s mentioned, he curls into the fetal position and sings opera. It’s fine until the two characters are ambushed and one of the enemies mentions pineapples. The two are captured because of the sidekick’s problem.
Is this the time for the main character to kindly overlook her friend’s defects? Nope. Now it’s a big problem of plot and character. That mystery needs to be solved.
But if the sidekick just makes a passing reference to a traumatic experience with pineapple, we have no problem with it. It never truly affects the plot. You can sprinkle clues about the sidekick’s past through the story as much as you want, but as long as it never affects the plot or character development, it’s an extra for the readers, not a mystery to be solved.
This is a useful, but rare, technique. Normally, you will introduce mysteries and solve them by the end of the book. If you promise a mystery’s solution, you must give the mystery’s solution. But if you know you can get away without solving the mystery for the reader, make sure you don’t promise its solution. Give the reader some extra credit homework. Not all of them will go for it, but some will. It may just add an unasked-for element of depth to your story.