  Let $\Psi$ be an endomorphism of a nilmanifold $M = N/\Gamma$. We show that
the preimages of a point under $\Psi$ become dense exponentially if and only if
$\Psi$ is totally non-invertible, which means $(M, \Psi)$ has no invertible
factors of quotients of factors. As an application, let $f$ be a non-invertible
Anosov covering map on $M$ with one-dimensional stable bundle, if $f$ has such
linear part $\Psi$ and is topologically conjugate to $\Psi$, then every
periodic point of $f$ admits the same stable Lyapunov exponent with $\Psi$, and
hence the conjugacy is automatically smooth along each stable leaf. Conversely,
if $\Psi$ is horizontally irreducible, which means the induced horizontal toral
endomorphism is irreducible, then $f$ admitting a constant periodic stable
Lyapunov exponent implies that there exists a topological conjugacy to $\Psi$.
  In summary, for an Anosov covering map on a nilmanifold with one-dimensional
stable bundle and totally non-invertible, horizontally irreducible linear part,
we get the equivalence among the existence of invariant unstable bundle, the
existence of topological conjugacy to linear part, and a constant periodic
stable Lyapunov exponent.
