  We present high-resolution measurements of the $c^\star$-axis thermal
expansion and magnetostriction of piperidinium copper bromide \hp. The
experimental data at low temperatures is well accounted for by a two-leg
spin-ladder Hamiltonian. The thermal expansion shows a complex behaviour with
various sign changes and approaches a $1/\sqrt{T}$ divergence at the critical
fields. All low-temperature features are semi-quantitatively explained within a
free fermion model; full quantitative agreement is obtained with Quantum Monte
Carlo simulations.
