Which Dune book has Paul explain how he sees the future with the metaphor of looking from peak to peak across a series of dunes, with the implication that he cannot see into the wells of the dunes?
This is from Dune Messiah; Alia and Paul have a conversation with Stilgar:
“Stilgar,” Alia said, fighting to hold him, “you stand in a valley between dunes. I stand on the crest. I see where you do not see. And, among other things, I see mountains which conceal the distances.”
“There are things hidden from you,” Stilgar said. “This you’ve always said.”
“All power is limited,” Alia said.
“And danger may come from behind the mountains,” Stilgar said.
“It’s something on that order,” Alia said.
Stilgar nodded, his gaze fastened on Paul’s face. “But whatever comes from behind the mountains must cross the dunes.”
You might also be thinking of the preface to "Arrakis Awakening" (from the original Dune novel)
"Muad‘Dib could indeed see the Future, but you must understand the limits of this power. Think of sight. You have eyes, yet cannot see without light. If you are on the floor of a valley, you cannot see beyond your valley. Just so, Muad’Dib could not always choose to look across the mysterious terrain." - — from “Arrakis Awakening” by the Princess Irulan