When I was still playing e4, I was striving for a complete repertoire. I tried to have prepared an answer for each black move in the opening. After the switch to 1.d4 I didn't manage anymore. It was and is too much. Because of that I chose a more experimental approach. I knew more or less the mainlines but the more subtle and not so subtle details would be investigated in the games. Of course this was risky. More than once I had with white! after 10, 12 moves a worse position and had to work hard for weeks even months to get a half point out of the game. Admittedly , those were very instructive experiences. But such was not the purpose. For a good result it is necessary to create with white winning chances.
So I bought the necessary books, of which a few repertoire-books. The pros and cons are known. I don't want to discuss this here. This time I want to tell about a recommendation which I don't really know how to judge properly. The recommendation is from GM Schandorff. His two books got good reviews and I must confess that he understands more about chess than I. It concerns the Old-Indian opening.
But what I have to think exactly of the Sämisch against the Old-Indian opening is still not clear to me. Next time I will just try it again.
For the ambitious beginner I can recommend 1.e4 d6 2.d4 Nf6 3.Nc3 g6 4.f3 (I don't care if this is imprecise) Lg7 5.Le3 O-O 6.Dd2 c6 7.O-O-O to play as I did for years. Without the experience I likely didn't win that game. Therefore I want to end with a light rapid-game, once again against a stronger opponent:
Black didn't understand the difference between the Pirc and the Kings-Indian.