Finished Star Trek: Assignment Earth. Pretty good. Wasn't there a second one of these?
You mean the IDW miniseries? No, that was the only one, although Byrne did sequels to one of its issues in his miniseries Crew
and Leonard McCoy: Frontier Doctor
. (Those three miniseries and the stories collected in Romulans: Pawns of War
form a sort of interconnected "Byrneverse" of their own.)
There were a couple of earlier Gary Seven stories in DC's TOS volume 2 by Howard Weinstein, unconnected to the Byrne comics. And of course there are the Greg Cox novels about Gary and Roberta.
For the record, Gary and Roberta are featured prominently in three of my novels: Assignment: Eternity, The Eugenics Wars (Volume One),
and The Eugenics Wars (Volume Two).
You might also check out In History's Shadow
by Dayton Ward.