I wouldn´t care too much about theoretical soundness of QGA,
it´s at least as "sound" as KID, Dutch, Benoni, Volga, Chigorin etc.
To my knowledge it´s the only defence against 1d4 that has
been played by ALL World Champions in serious games!
The four books mentioned by Templare2 are certainly the best ones.
I recommend to start with Raetsky-Chetwerik. It´s the best for your
level and has a more complete coverage than Rizzitano or Sakaev.
It´s not as deep as those but a better introduction: I really like it
when Rartsky/Chetverik cover Steinniz´ old 6...cd4 before turning to
the modern 6...a6 . Besides, in the most critical systems 7Bb3 and 3e4
all these books are somewhat outdated and you have to work through
the most recent games. Though, if you stay with the QGA after working
through Raetsky/Chetverik, you should add at least one of Rizzitano
or Sakaev for the big theory in several main lines.
A general problem is that white has a wide choice of dangerous attempts
with totally different kinds of play. Black can deal with each of these
but especially the exchange system 7dxc5 is not to everyone´s taste:
like french exchange or slav exchange you either love it or hate it!
Further, QGA is no stand-alone defence against all white closed openings:
it´s a classical and logical defence well worth being added to your armoury
but probably no more. And: when you don´t "feel" Slav or QGD positions,
I´m not sure if QGA is the right decision. They are somewhat similiar.
(Especially the Semi-Slav Meran is something like an improved QGA).
tracke