  We consider a two-layer Heisenberg antiferromagnet which can be either in the
N\'{e}el-ordered or in the disordered phase at $T=0$, depending on the ratio of
the intra- and interlayer exchange constants. We reduce the problem to an
interacting Bose-gas and study the sublattice magnetization and the transverse
susceptibility in the ordered phase, and the spectrum of quasiparticle
excitations in both phases. We compare the results with the spin-wave theory
and argue that the longitudinal spin fluctuations, which are not included in
the spin-wave description, are small at vanishing coupling between the layers,
but increase as the system approaches the transition point. We also compute the
uniform susceptibility at the critical point to order $O(T^2)$, and show that
the corrections to scaling are numerically small, and the linear behavior of
$\chi_u$ extends to high temperatures. This is consistent with the results of
the recent Monte-Carlo simulations by Sandvik and Scalapino.
