Mazukevich's Madness: A repertoire for Black against the Four Knights. Opening Hacker Files, #14

In Mazukevich's Madness, Black stuns White with a piece sacrifice on move 5, completely changing the direction of the game of this otherwise solid and rather predictable opening. FIDE Master, FIDE Trainer, and opening specialist Carsten Hansen carefully analyzes all options for White after 1.e4 e5 2. Nf3 Nc6 3. Nc3 Nf6 4.d4 exd4 5. Nxd4 Nxe4!, showing that Black not only is doing fine, but can often start pushing for more if White doesn't play accurately.
In addition to this analysis, in the second half of the book, Hansen presents a complete repertoire for Black against all other lines of the Four Knights, including the main lines 4. Bb5 and 4.g3, the modern 4.h3, 4.a3, and 4.a4, the sharp Belgrade Gambit, a favorite of Naroditsky's 4. Nd5, as well as other minor lines for White.