  We present extensive early photometric (ultraviolet through near-infrared)
and spectroscopic (optical and near-infrared) data on supernova (SN) 2008D as
well as X-ray data analysis on the associated Swift/X-ray transient (XRT)
080109. Our data span a time range of 5 hours before the detection of the X-ray
transient to 150 days after its detection, and detailed analysis allowed us to
derive constraints on the nature of the SN and its progenitor; throughout we
draw comparisons with results presented in the literature and find several key
aspects that differ. We show that the X-ray spectrum of XRT 080109 can be fit
equally well by an absorbed power law or a superposition of about equal parts
of both power law and blackbody. Our data first established that SN 2008D is a
spectroscopically normal SN Ib (i.e., showing conspicuous He lines), and show
that SN 2008D had a relatively long rise time of 18 days and a modest optical
peak luminosity. The early-time light curves of the SN are dominated by a
cooling stellar envelope (for \Delta t~0.1- 4 day, most pronounced in the blue
bands) followed by 56^Ni decay. We construct a reliable measurement of the
bolometric output for this stripped-envelope SN, and, combined with estimates
of E_K and M_ej from the literature, estimate the stellar radius R_star of its
probable Wolf-Rayet progenitor. According to the model of Waxman et al. and of
Chevalier & Fransson, we derive R_star^{W07}= 1.2+/-0.7 R_sun and
R_star^{CF08}= 12+/-7 R_sun, respectively; the latter being more in line with
typical WN stars. Spectra obtained at 3 and 4 months after maximum light show
double-peaked oxygen lines that we associate with departures from spherical
symmetry, as has been suggested for the inner ejecta of a number of SN Ib
cores.
