Just re-read this:
Another way to enjoy The Lord of the Rings: The Card Game solo is by controlling multiple decks to learn how each sphere of influence interacts with each other.
Which makes it sound like you are running the game as 2 players.
Since decks can contain any combination of cards, trying each sphere out and seeing how they play off others is a good way to determine which spheres would make up your ideal deck.
Which brings me back to confusion as far as how the deck-building goes. Sounds like each deck could have up to 3 different spheres in it, since you can have up to 3 different heroes.
Ugh, just need the rules already!