We use high dynamic range, high-resolution L-band spectroscopy to measure the radial velocity (RV) variations of the hot Jupiter in the τ Boötis planetary system. The detection of an exoplanet by the shift in the stellar spectrum alone provides a measure of the planet's minimum mass, with the true mass degenerate with the unknown orbital inclination. Treating the τ Boo system as a high flux ratio double-lined spectroscopic binary permits the direct measurement of the planet's true mass as well as its atmospheric properties. After removing telluric absorption and cross-correlating with a model planetary spectrum dominated by water opacity, we measure a 6σ detection of the planet at K_p = 111 ± 5 km s^(−1), with a 1σ upper limit on the spectroscopic flux ratio of 10^(−4). This RV leads to a planetary orbital inclination of i=45^(+3)_(-4)° and a mass of M_p = 5.90^(+0.35)_(-0.20)M_Jup. We report the first detection of water vapor in the atmosphere of a non-transiting hot Jupiter, τ Boo b
