I'm working on the final installment of Willa Fox's adventures right now and hitting the typical Third Book Problems. By the time you write up to the end of a trilogy, the plot should thicken, the emotional stakes should be raised even higher and it all needs to resolve in a satisfying way that does the previous two books justice. Yeah, no biggie.
One of the trickiest things I find about maintaining the mystery at this stage is knowing too much. Obviously, as the author, you know exactly what's going on and how things will end up, and all the intricate backstory that explains everything. But sometimes that knowledge can be a burden. I find that in the early drafts I tend to lead the reader right to the X that marks the no doi. It's only in the revision process that I can start to detach, complicate the situation, create some red herrings or some incorrect assumptions.
Struggling with knowing too much? Here are a few tricks I have learned and some questions to ask yourself:
1) How could a reader misinterpret the existing clues? Can your MC get tripped up by a detail or two?
2) Is there dialogue that can be more elliptical? People don't always speak in a literal manner and your characters shouldn't, either. Can you play up a misunderstanding?
3) Are there mini mysteries within your overarching mystery that can be teased out to distract your reader and your MC from the truth?
4) What qualities, characteristics or motivations can you add that might complicate the reader's opinion of the possible suspects?