Which leads should I pursue?

Hint 1 of 11:

I'd start by talking to David Sennett, Armstead's butler.

Hint 2 of 11:

Maybe you should go to the French Embassy to see Gerard, Armstead's friend.

Hint 3 of 11:

Also drop by the Bridgehouse Hotel to see Pierre Matin, Armstead's source
for the book.

Hint 4 of 11:

It wouldn't hurt to talk to each of the survivors of the Tontine lottery.

Hint 5 of 11:

If you're interested in what's written in the book, drop by Norgate and
Company, the publishers.

Hint 6 of 11:

The publisher will talk more about a Russian man, Rostov.  I would check to
see who this man is.