This is something I was asked after my Musashi article and my answer is very much at odds with the mainstream view. As I see the opening as little more than a prequel to the middle game so I think the major focus should be on which middle games someone should be willing to play.
There is a problem here too in that understanding these middle games will be largely a function of where someone is with their chess development. So if a player is still developing their basic tactical play they should look for positions in which the emphasis is primarily on piece play. As they then acquire ever more sophisticated and subtle chess patterns there will be an ever greater emphasis on pawn play, starting with relatively fixed structures and standard plans and gradually moving up the scale of difficulty to flexible positions. If someone misses out one of these stages of development they’ll never quite get it right. So a degree of objectivity is required about what we are capable of.
Had I had a good coach during my teenage years my own development would have been different. I moved into highly flexible Flank Openings too quickly, missing out the important stage of playing more standard positions. A lot of players tend to make the same mistake through no fault of their own. Confronted with a bewildering mass of (mis)information and little in the way of objective guidance it’s too easy to get into openings that are way too complex for one’s level of understanding. In my case I figure it knocked 100 or so Elo points from my upside potential.
There are other aspects to this choice of openings and for the vast majority of people I must strongly recommend that they go for those which place a high degree of emphasis on understanding rather than memory. What this means is that if you are at the right level to understand the plans and ideas then you should be able to play it based largely on this alone. This maximizes the amount of time available for important things like the endgame.
Assuming the choices are good we finally get to the ‘preparation’ part. How should one do this? The most efficient way, in my opinion, is to play through some well annotated games in which this particular opening was played in order to deepen one’s understanding of the structure and typical middle game positions. After that you go out and play it, perhaps choosing internet or correspondence games at first. You then go through these, possibly with your coach, and gradually some specific lines of play will start to crystallize out of your efforts. These will then be founded upon a firm basis of positional understanding.
What does all this have to do with the vast majority of openings books? Not very much, though Everyman tends to be closer to the ideal of massive strategic explanation and well annotated games.