The package arrived in a green sack with the name DARCY stenciled on the front flap and a mystical sigil on the back.
The Limited Edition of Bookhounds of London in faux-leather,
Hardcover edition of the Liber Fumo de Frater Vigilio (The Book of the Smoke, also known as the Occult Guide to London)
A pamphlet, giving the in-universe background to the mystery of Mr. Darcy's death (and credits)
Period ephemera; props from the '30s, such as booklets, maps, postcards
A clue, part of the mystery of Agustus Darcy's death.
I won't add very much at this stage, except to say that each copy of the Limited Edition of Bookhounds of London can be identified by a different Lovecraftian adjective rather than a number and the each package contains different real Thirties ephemera. There is a thread on Yog-Sothoth.com collecting information about the different packages with speculations about their meaning.