This defense used to be my main black weapon and I occasionally still use it. The Bauer ('Philidor Files') book is the best source I am aware of.. Also fun - but not so solid - are the recommendations given in the 'The Black Lion' book..
The absolute main line is:
1.e4 d6,2. d4 Nf6, 3.Nc3 Nbd7, 4.Nf3 e5, 5.Bc4 Be7, 6.0-0 0-0, 7.Re1 c6, 8.a4 b6 and black plays b6,Bb7 while white prepares to play d5 in the most desirable circumstance.. The evaluation is a slight += but black usually manages to equalize if white's name isn't Anatoly
The g4 gambit is not refuted by Bauer and I can't imagine any other book that could have its refutation in it.. I haven't seen the Shirov video but I suspect that the so called refutation leads to unclear/equal position and not to a black advantage.
The f4 lines are very dangerous to the unprepared - black has to play some "only moves" to stay alive - see Bauer for more.
I used to avoid the f4 lines by playing e5 at the 3rd move before Nbd7. This has the disadvantage that it allows a transposition to an ending that visually looks very advantageous for white..
1.e4 d6, 2.d4 Nf6, 3. Nc3 e5, 4.de de, 5.Qxd8 Kxd8, 6.Bc4
when both 6.. Ke8
and 6... Be6, 7.Bxe6 fe6 and black will develop Bd6, Ke7 and start a queenside pawn storm
I prefer the latter (see Zurab Azmaiparashvili games and the Bauer book for more on this ending)
The Ng5-e6-c7-a8 line mentioned in this thread leads to a Black advantage after 10...b5! and white is in trouble - see the Bauer book