OK, I've thought a bit more about the enclosed subordinate.
I'll repost this, as the original is locked
Your book is right, IMO.
... since we learned that the boss gave him a job
is
a subordinate, and in subordinates the tense simplification is
recommended (see Swan, Practical English Usage, Tense simplification).
In this case the expected
past perfect is simplified to past.
As a proof, at Google, the hits are:
32 for "since the boss gave"
0 for "since the boss had given"
Not only that, but the tense sentence in:
Nobody has criticized Billy Joe since we learned that the boss gave him a job ...
is clear, IMO.
It's
normal to have a moment in the past (defined by simple past) followed
by an action leading to the present (described with present perfect.)
EDIT: My doubts wrt my original posting were caused by the fact that in:
... since we learned that the boss gave him a job
we're dealing with a subordinate
that the boss gave him a job
nested within another subordinate
since we learned
I wondered if
had given can be simplified to
gave in
this nested situation. However, I've decided that once one starts tense
simplification, one should proceed with it to all levels nested within.
Bottom line: I think both versions are correct. If you really want to emphasize the time sequence, don't "tense simplify."