  The discovery of the almost ideal, two-chain spin-ladder material
(C_5H_12N)_2CuBr_4 has once again focused attention on this most fundamental
problem in low-dimensional quantum magnetism. Within the bond-operator
framework, three qualitative advances are introduced which extend the theory to
all finite temperatures and magnetic fields in the gapped regime. This
systematic description permits quantitative and parameter-free experimental
comparisons, which are presented for the specific heat, and predictions for
thermal renormalization of the triplet magnon excitations.
