Now I gotta remind you, (and you know this) there will never be "one" of anything. You will have more than one type of fly; one type of rod, etc.
However, at this level, since you need a nice intro to the style and also.......the technique of the fishing, reading ideal spey water, I'd recommend you consider Dec Hogan's "A passion for steelhead". It covers the whole essence of the two handed experience, along with some nice diagrams and overviews of each of the required casts.
Like, there's dudes who can cast, but then, really don't "fish" well? Instruction directly from a guides mouth is quite valuable.
Well worth it; a book that can be used as a tool and a coffee-table decoration. Best $60 bucks I've ever spent.
What I lack in skill --- I make up for with enthusiasm.