  We have measured the Li abundance of 18 stars with -2 <~ [Fe/H] ~< -1 and
6000 K <~ Teff <~ 6400 K, a parameter range that was poorly represented in
previous studies. We examine the Galactic chemical evolution (GCE) of this
element, combining these data with previous samples of turnoff stars over the
full range of halo metallicities. We find that A(Li) increases from a level of
\~2.10 at [Fe/H] = -3.5, to ~2.40 at [Fe/H] = -1.0, where A(Li) = log_10
(n(Li)/n(H)) + 12.00.
  We compare the observations with several GCE calculations, including existing
one-zone models, and a new model developed in the framework of inhomogeneous
evolution of the Galactic halo. We show that Li evolved at a constant rate
relative to iron throughout the halo and old-disk epochs, but that during the
formation of young-disk stars, the production of Li relative to iron increased
significantly. These observations can be understood in the context of models in
which post-primordial Li evolution during the halo and old-disk epochs is
dominated by Galactic cosmic ray fusion and spallation reactions, with some
contribution from the $\nu$-process in supernovae. The onset of more efficient
Li production (relative to iron) in the young disk coincides with the
appearance of Li from novae and AGB stars. The major challenge facing the
models is to reconcile the mild evolution of Li during the halo and old-disk
phases with the more efficient production (relative to iron) at [Fe/H] > -0.5.
We speculate that cool-bottom processing (production) of Li in low-mass stars
may provide an important late-appearing source of Li, without attendant Fe
production, that might explain the Li production in the young disk.
