If you answer 1 e4 with 1...e5, you'll almost certainly have to face the Four Knights with Black. And this book will help you learn it. After 1 e4 e5 2 Nf3 Nc6 3 Nc3 Nf6, you will learn what to do against:
4 a3 d5! (a Scotch Four Knights with colors reversed!)
4 Bc4? Nxe4! (you can also get here by trying to play a Two Knights defence with Black).
4 g3 (the Glek system) 4...d5 (4...Bc5 is also given a chapter)
4 d4 exd4 5 Nd5 (the Belgrade gambit) 5...Be7
4 d4 exd4 5 Nxd4 Bb4 (the main line of the Scotch Four Knights)
4 Bb5 Nd4 (4...Bb4 is also given a chapter), the Spanish Four Knights. Actually, Pinski shows that 4...Bd6 is also quite playable here. One of the more unusual variations he gives here is 4...Bd6 5 g4 Bc5 6 g5, but I'd certainly much rather have Black in this position.
Of course, this book will also help you play the Four Knights with White. And that can come in handy, even if you play the Ruy. When I was a beginner, decades ago, I decided to play the Exchange Ruy with White. But the first time I tried it, it went like this:
1 e4 e5 2 Nf3 Nf6. I did not want to play against the Petroff Defence, so I decided to play something I knew (since I defended against 1 e4 with 1...e5), namely the main line of the Scotch Four Knights! 3 Nc3 Nc6 4 d4 exd4 5 Nxd4 Bc5 (oops, my opponent chose a rare sideline) 6 Nxc6 bxc6 7 Bd3 d6 8 0-0 and I had at least an equal game, and maybe a slight edge. In any case, I was never in trouble and eventually won. To my surprise, as this book relates, this exact position was reached many years later in the game Miles versus Hebden.
In my next attempt to play the Exchange Ruy, the game went:
1 e4 e5 2 Nf3 Nc6 3 Bb5 Nf6. (Oh, no! That's my beloved Berlin Defence! I decided to avoid it with White, and try the Spanish Four Knights instead.) 4 Nc3 Bb4 5 0-0 0-0. (Now I played a rare sideline myself, to try to get my opponent out of his book.) 6 Re1 (this move is not in Pinski's book, but it is playable) 6...d6 7 Bxc6 bxc6 8 d4 exd4. (I was getting worried here. If I played 9 Nxd4, Black would probably be in good shape after 9...Bxc3. So I considered playing 9 Qxd4, but I eventually decided against it.) 9 Nxd4 Bb7 (It seems that Black got the idea of playing Bb7, c5, and Bxc3, winning my e-pawn, but he did not execute this plan very well.) 10 Bg5 (defending against this threat) 10...h6 11 Bd2 (As you will see, I am still defending against the threat.) 11...Bxc3 12 Bxc3 c5 (chasing my Knight to where I wanted to play it anyway) 13 Nf5 Bxe4? (Now Black gets slaughtered.) 14 Rxe4 Nxe4 15 Bxg7 (15 Qg4 is even better) 15...Qg5 16 Qf3 Nd2 (if 16...Rfe8 17 h4) 17 Qd5 c6 18 Qd3 Rfe8 19 Bxh6 Qh5 20 Bxd2 Re5 21 Qg3+ Qg6 (if 21...Kf8 22 Bh6+) 22 Qxe5 Black Resigns
I recommend this book, and I advise all chess players to learn about the Four Knights.