We have measured the Li abundance of 18 stars with -2 ~ { } ^ { < } _ { \sim } [ Fe/H ] ^ { < } _ { \sim } ~ { } -1 and 6000 K ^ { < } _ { \sim } ~ { } T _ { eff } ~ { } ^ { < } { } _ { \sim } 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 ~ { } \sim 2.10 at [ Fe/H ] = -3.5 , to \sim 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 .