1.e4 d5 2.exd5 Qxd5 3.Nc3 Qd6!? (Dia)
In recent years this seems to overshadow the old mainline 3...Qa5. Quite recently there has been a second edition of Michael Melts' 'Scandinavian Defense: The Dynamic 3...Qd6'. The book is a treasure chest containing an enormous amount of well organized information on the line and lots of independent analysis. Unfortunately it's also very hard to navigate and contains very little prose or guidance except for a few introductory chapters.
For some minor (possibly insignificant) reasons I prefer 4...a6 - usually followed by 5...Nf6.
5.g3!? is an alternative move-order with some independent ideas.
I find this a much more attractive move than ...c6. Black may follow up with ...Nc6, ...Bg4 and 0-0-0 but also ...b5, ...Bb7 and ...e6.
This seems to be the new mainline. White not only makes ...b5 less attractive but also prepares Bf4.
7.h3 is another important option.
Black also needs to prepare for the immediate 8.d5.
Again the immediate 9.Bf4 must be considered.
This is Melts recommendation (Game 18, page 151, line B2e2d2!) and indeed the move is starting to look forced:
a) 9...Nxd5 10.Nxd5 Qxd5 11.Qxd5 Rxd5 12.Ng5 of Lakos-R.Perez, Ortigueira 2002 is entirely unattractive for Black.
b) 9...Nb4 is a more optimistic approach, forcing White to choose between a repetition and complications. Unfortunately the complications seem close to winning for White: 10.h3 Bh5 11.Bf4 Qc5 12.Be3 Qd6 13.Qe2! Nbxd5 14.Nxd5 Nxd5 15.Rad1! Qf6 and White has a pleasant choice:
b1) 16.Bd4 Qe6 17.Qxe6+ fxe6 18.Be5 Bxf3 19.Bxf3 c6 20.Bg4 Nc7 21.Rxd8+ Kxd8 22.Rd1+ +- Ibarra Jerez-Trent, Chalkidiki 2003.
b2) 16.c4 Nxe3 (16...Nb4 17.Rxd8+ Kxd8 18.Rd1+ Kc8 19.Qd2 Qd6 20.Qe1 Qf6 21.g4 Qxb2 22.Rb1 +- Humphrey-Aplin, Kuala Lumpur 2006) 17.Rxd8+ Kxd8 18.Qxe3 c6 19.g4 Bg6 20.Qb6+ Kc8 21.Rd1 e5 22.Nxe5 +- Rasik-Antoniewski, Czechia 2006.
The queen sacrifice 10.Nxe5!? is enterprising but on closer scrutiny doesn't seem too terrifying:
a) 10...Qxe5 11.f3 Be6 12.Re1 Qf5 13.f4 Qg6 14.Re5 Bf5 = Ninov-Panbukchian, Pleven 2005.
b) 10...Bxd1 11.Nxf7 Bxc2 12.Nxd6+ exd6 13.Re1 Re8 14.Be3 g6 15.Rac1 Bf5 16.Ne2 Bg7 17.Nd4 Ng4 18.Nxf5 gxf5 = Ragger-Nikolov, Kranj 2004.
10...Nxf3+ 11.Bxf3 Bxf3 12.Qxf3 e5 13.dxe6 Qxe6 14.Bg5 (Dia)
This seems to be the crucial position. Black has tried a number of different moves but none seem to give full equality:
a) 14...Qc6 15.Qxc6 bxc6 16.Bxf6 gxf6 17.Rad1 Bb4 18.Ne2 Bd2 19.Kg2 +=
b) 14...h6 15.Rfe1 Qb6 16.Nd5 (16.Bxf6 Qxf6 17.Qxf6 gxf6 18.Re4 Bc5 19.Rae1 Rd2 20.R1e2 Rxe2 21.Rxe2 Bd4 22.Re7 Bxc3 23.bxc3 += Tukhaev-Vasiliev, Evpatoria 2006) 16...Rxd5 17.Bxf6 Qc6 18.c4 Rd6 19.Qxc6 Rxc6 20.Re8+ Kd7 21.Rd8+ Ke6 22.Bc3 +/- Mardell-Brandt, Taby 2007.
d) 14...Bd6 15.Rfe1 Be5 16.Ne4 (16.Re2 Rde8 17.Rae1 Nd7 18.Bf4 f6 19.Qe3 g5 20.Bxe5 Nxe5 = Stiri-Dounis, Athens 2007; 16.Bf4 Nd7 17.Nd5 f6 18.c4 g5 19.Bd2 h5 20.Ba5 Qg4 = Williams-Hamad, Turin 2006) 16...Rhe8 17.Nc5 Qd5 18.Qxd5 Rxd5 19.Bxf6 gxf6 20.Nd3 Re6 21.f4 Bd6 22.Rad1 += Huerga Leache-Garcia Paolicchi, La Massana 2008.
c) 14...Bb4 15.Rfe1 Qb6 16.Bxf6 gxf6 17.Nd5 (is 17.Rfe1 better?) 17...Rxd5 18.Qxd5 Bxe1 19.Rxe1 Qxb2 20.Qxf7 Kb8 = Pesotsky-Bazarov, Lipetsk 2008.
e) 14...h5 15.Bxf6 (15.Rfe1 Qg4 16.Qxg4+ hxg4 17.Ne4 Nxe4 18.Rxe4 f6 = Sedina-Danielian, Elista 2004) 15...Qxf6 16.Qxf6 gxf6 17.Rad1 Bd6 (17...Bc5 18.Ne4 Be7 19.Rfe1 Rhe8 20.Nc3 c6 21.Rxd8+ Kxd8 22.Kg2) 18.Nd5 h4 19.Kg2 Rh5 20.b3 b5 21.Rfe1 += Fernando-Galego, Vila Real 2005.
This line may attract a (semi) professional player who feels confident that he can hold a draw against well prepared opponents in one of the lines after 14.Bg5. However, for the average club player (who represents the main segment of chess book buyers) defending a slightly inferior endgame like this for several dozens of moves sounds like a nightmare and will not be a good selling point.