Question: I remember reading a book in the 70s, so it could be YA. It is set on modern-day Earth, and the story is about a boy. His mother is sick (maybe cancer?) and he is socially awkward, so he doesn't have much self-esteem. He stumbles upon an old building (maybe a barn?) and somehow meets a person/creature who tells him that the King of Light (or maybe the King of Sun) and the Queen of Dark (or maybe the Queen of Moon?) have been at war with each other for a long time, each wanting to take over. He is told he has been given the choice over who will rule.

The boy thinks the Queen is beautiful, so he chooses her. Everything feels calm, and people who were mean to him are no longer mean. However, as he goes about his day, he sees that people become more and more sad, depressed, and lethargic. His mother in particular becomes sullen and morose. No one has any energy. Frantic, he runs back to the space and says that he wants to make a different choice...

So, the boy chooses Light. This time as he goes about his day, he sees his mother having more energy to work around the house, people are accomplishing things, etc. After a while, however, he sees that people are becoming too angry and too rough. There is too much energy and people start fighting.

He goes back to the place, frightened and confused. He says that both choices brought negative reactions, so both choices were wrong. If both choices were wrong, how could he choose?

The person/creature tells him that there is a right choice. The boy is confused. The person/creature tells him that he had another choice. He could choose to do nothing.

And that is what he chooses.

So, balance is restored and the war between Light and Dark continue. The moral is something like “balance is necessary and one must fight to keep that balance.”

I've always wanted to find the book again, but I cannot remember a title nor an author. If anyone knows what book this is, I will be eternally grateful.

Answer: 
Riker, Geordi, Troi, and Data all have bonuses going for them:

* Riker is great at bluffing, and has rather good luck
* Geordi could see through the back of the cards
* Troi is an empath, and can generally tell when someone is bluffing
* Data has memorized an immense number of games and can easily calculate probabilities

But they also tend to check and balance each other in various ways:

* Troi can't read Data
* Data starts out unable to tell if anyone's bluffing, but gets better as the series goes on
* They usually play with cards that have a special backing that Geordi can't see through (said to Worf after a game once)
* Troi and Riker were once quite close, and still retain the empathic link - she can almost certainly tell when he is bluffing, even if she has some difficulty with the others
Data seems the most able to fill that role given that he is shown to clean house with the players in "Time's Arrow". He mentions at some point during the series that he initially memorized virtually every book on the subject but found that the actual game was different than the mere "book" knowledge. Given that he can simulate games in his head with actual data culled from past games as well as being able to precisely recall each player's appearance or gestures at any given moment and weight that against the numerical performance of that player and the game as a whole.