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.