Book on 3...c6 in Pirc
Well I had this played against me so I'm no great expert or anything but a few things pop out. One is that ...c6 prepares ...b5 which could possibly then go to ...b4, attacking White's Nc3 which is defending his e4 pawn. Very crude.
...Qa5 is also a possibility and the pin on the knight again makes ...Nxe4 a threat. Or there is ...Qc7 in support of a central thrust with ...e5
Finally Black can always play ...g6 and transpose back to the Pirc -- presumably he only does this when it's a line where ...c6 is known to be advantageous to Black. I don't know which lines those are though.
Another resource (especially if playing as white against the Czech Defense) is the reviews posted on amazon for both of these books. Some of these reviews question the soundness of this opening for black, and offer specific lines for white. I'm not yet convinced that there is a definite refutation, but the lines offered there are probably worth looking into for anyone playing this opening, or playing against it.
I doubt it's unsound, not at the amateur level anyway