  For a system of two-chain spin ladders, the ground state for weak interladder
coupling is the spin-liquid state of the isolated ladder, but is an ordered
antiferromagnet (AF) for sufficiently large interactions. We generalize the
bond-operator mean-field theory to describe both regimes, and to focus on the
transition between them. In the AF phase near the quantum critical point (QCP)
we find both spin waves and a low-lying but massive amplitude mode which is
absent in a conventional AF. The static susceptibility has the form $\chi(T) =
\chi_0 + a T^2$, with $\chi_0$ small for a system near criticality. We consider
the dynamical properties to examine novel features due to the presence of the
amplitude mode, and compute the dynamic structure factor. LaCuO$_{2.5}$ is
thought to be such an unconventional AF, whose ordered phase is located very
close to the QCP of the transition to the spin liquid. From the N\'eel
temperature we deduce the interladder coupling, the small ordered moment and
the gap in the amplitude mode. The dynamical properties unique to near-critical
AFs are expected to be observable in LaCuO$_{2.5}$.
