The Lightner Double
The Lightner double calls for dummy's first bid suit. Well, at least that's what you're likely to hear at first. Then someone will come along and say, No, it calls for an "unusual lead". The problem with that is that "unusual" is a little vague and ambiguous. What's unusual to you won't necessarily be all that unusual to your partner. So I would advise, well, until you become a nationally recognized expert, or maybe when you've got an awfully good rapport with your partner, to stick to the totally unambiguous first interpretation. There will be enough opportunities to employ it on that interpretation. It comes in handy when you've got a void in the suit bid by your RHO and the opps are in a trump contract. Oh, but wait a minute. It's supposed to mean that you can beat the contract on that lead, not that you promise one trick. So it's not enough against little slam unless you have a reasonable presumption of another trick. Above are links to some illustrations.
Having come back to this, I decided to look up what the redoubtable Eddie Kantar had to say about the Lightner Double, and as you might have expected, he had a few more words than I've offered. He lists four situations and gives his advice, starting not with what to do but with prohibitions:
A. If the Doubler has Bid a Suit:
1. Partner is forbidden to lead that suit.
2. Partner is forbidden to lead a trump.
3. Partner should assume that the double was either based upon a void suit or an unexpected A K (or A Q) in a suit bid by the opponents. (Very often dummy's first bid suit.)
And he ends his page and a half with this piece of advice: When in doubt lead dummy's first bid suit, although the double of a suit slam is in no way a command to do so.
So he doesn't come down hard on "dummy's first bid suit", but he does refer to that idea several times, including the near catch-all "when in doubt". The reader is encouraged to look up Kantar's full monty on this matter, but in the meantime, I would only repeat the above suggestion that you'd do well to abide by the unambiguous "dummy's first bid suit" until you get real good and you have an exceptionally fine rapport with your partner.