The Leerans can read other people's minds. But they can't do it if they're too far away from the being they're trying to read. What is the maximum distance from which they can read minds clearly?
Probably between a few feet and a few yards
It's not entirely clear (and potentially inconsistent) just how far a range the Leeran mind-reading ability works at, but Ax provides his best guess in book 15: The Escape.
I took a deep breath. "Anyway, Ax says these Leerans are psychic. So we have to be very careful. We can't get near one of them."
"How near is near?" Jake asked Ax.
<I don't know,> Ax admitted. <I think a few feet. But I don't know.>
However, in book 18: The Decision, a group of Leeran-Controllers are able to determine the identities of the Animorphs from a distance at which ranged weapons were a reasonable option but somewhat capable of being dodged, which seems to imply it's a bit further than "a few feet" would really apply to.
The spear flew through the water only slightly slower than a human bullet. I jerked left. Too late! The spear pierced my tail and kept on flying.
<Hey!> Marco yelled.
<I'm an Andalite! Andalite!> I cried. <Your friend! Your ally!>
<Aximili-Esgarrouth-Isthill and three humans from planet Earth. Not our allies,> a cold, thought-speak voice said. He laughed. <You have no secrets from these psychic Leeran minds.>
And suddenly the water boiled with the firing of a dozen spears.
This time we were more prepared. Still, we were not fast enough. A spear hit me in the side and stuck. Prince Jake avoided being hit, but Cassie was speared through and through. Marco was hit twice. Shark blood billowed.