  We present an improved direct measurement of the parity-violation parameter
A_b in the Z boson-b quark coupling using a self-calibrating track-charge
technique applied to a sample enriched in Z -> b b-bar events via the
topological reconstruction of the B hadron mass. Manipulation of the SLC
electron-beam polarization permits the measurement of A_b to be made
independently of other Z-pole coupling parameters. From the 1996-98 sample of
400,000 hadronic Z decays, produced with an average beam polarization of 73.4%,
we find A_b = 0.906 +/- 0.022(stat.) +/- 0.023(syst.).
