I've already spent several posts talking about the story arc and the initial scenarios in a campaign, and I don't intend to repeat myself here. Instead let's dive right in and talk about specifics for this campaign.
Bookhounds is all about trade. The characters will be buying and selling books, and trying not to die or be driven mad while doing so. They will attend book auctions - they may even host some auctions of their own - and they will be dealing with the hideous unappeasable monster that is the British Public, as they try to get customers in the door. The arc needs to reflect this, and the initial scenarios should be designed so as to show the players how things are done in this new world.
Also, this being a Technicolor landscape, the initial scenarios need to play up to the Hammer Horror stereotypes. So let's add a bit to the mix and say that the scenarios ought to draw inspiration from Hammer's films, and as there are so many of those I'm going to arbitrarily select from this top ten list and see what happens next.
Let's start with Demons of the Mind, shall we?
In that one a worried father, convinced his children will go insane, locks them away in his house and brings in a doctor, played by Patrick Magee, to take care of them. Meanwhile strange, inexplicable murders take place near the house. Are one of the children - both of the children, perhaps - responsible? Is some other force at work? The childrens' mother, now dead, was a hopeless homicidal lunatic, which is why the father's so sure his son and daughter will go the same way. The Cultes des Ghoules is all about necromantic communion, and here we have the beginnings of a plot, since there's at least one copy of that book hidden somewhere in du Bourg's.
Now, as covered in previous posts, the first session of a new campaign ought to start low key. The objective is to introduce the characters to the world and its mechanics, and then plunge them into situations in which their new knowledge will help them get out alive again. So the very first scenario ought to be all about du Bourg's, since this location will become the most important aspect of the ongoing narrative. This scenario will also introduce the main antagonist for the first arc: Rupert Hardy, Baron Stavesham.
The Baronetcy of Stavesham goes back to 1086, and has its manor in Monmouthshire. It has its share of black sheep and rakehells, but the current Baron is supposed to be a thoroughly ordinary fellow with an unfortunate past. His wife Susannah, a great beauty, died in a train wreck in 1917, leaving him to bring up two children: Eric and Anna, both now in their late teens. Nobody has seen either child in some time; their father, it is said, has unusual views on parenting. This fact would require a 1 point pool spend, Keeper's choice as to which: the very best alienists have been summoned to his house in Piccadilly to treat his children, in conditions of closest secrecy, but nothing has worked.
Clearly something odd is going on. The movie that is this arc's inspiration uses incest and buckets of blood, but as Keeper you may choose to go in a different direction. The whole thing's available on YouTube if you want to watch it.
This arc assumes that the father, Rupert Hardy, wants any information that might help cure his children's alleged condition. Whether that condition really exists or not is up for debate. However in order to get that information he'll go to any lengths, and that includes necromancy, to talk with his dead wife. Hence the Cultes des Ghoules, and his interest in du Bourg's.
I see this arc ending with a shattering conclusion to Hardy's story, and revelations as to the true nature of the affliction that dogs his son and daughter. Probably there will be several deaths in that family; maybe the endgame goes the full Usher and the final shot is of Hardy's burning London townhouse, with all the family inside. However Hardy's story ends, the Bookhounds finish with a copy of Cultes des Ghoules. The job of the next arc will be to decide what they want to do with it.
So assuming a three to four scenario first arc, I see the campaign going something like this:
First scenario introduces the characters, du Bourg's, and its day to day routine. If the Keeper intends to use any optional rules, like Idiosyncratic Magic or Megapolisomancy, those rules ought also to be introduced or at least hinted at here. Stakes should be low, but significant enough that the characters need to display a little ingenuity. In displaying ingenuity they probably also play up their characters' quirks and motivation - their Drives - which can only help future sessions. When I say low key, I mean something like this incident borrowed from Lutyens and Rubenstein: there's a big seasonal event about to take place, perhaps a Christmas party for the staff and important customers (hence why Hardy attends), but the weather's so atrocious that all the arrangements (and dinner preparations) are thrown into chaos. The characters need to step in to prevent disaster. In order to get the Auction ability involved, why not make it one of those trade dinners that were so popular with antiquarian bookstores once upon a time?
Second scenario develops on the first. Any optional rules introduced then need to be at the forefront now. If there's Idiosyncratic Magic, the characters need to start using it. If you intend for characters to have Magic pools, they need to start gaining points. At least one of du Bourg's significant secrets needs to be developed and possibly dealt with. Say the truth behind Mash, or what's really up in the closed-off rooms. Clues to the location of the Cultes des Ghoules need to be developed, but there ought to be no pay-off yet; the book will not be found until the third scenario. At least one or two NPCs will end up dead, though possibly not in or around du Bourg's. After all, there's still Hardy's London townhouse and those two crazy kids to consider.
Third scenario unravels Hardy's plot. The true nature of the crazy kids is revealed, as well as the real reason why Hardy's after the Cultes. That almost certainly ends in death for several people, possibly even a PC if the players have been incautious. Now's the time to find that copy of Cultes, but naturally there's a price to be paid for that as well, probably in blood. No doubt that mummified body in the basement of du Bourg's has a secret or two to reveal. After all, this is all about necromancy, and who better to deliver the final revelations than a man long dead?
Fourth scenario concludes Hardy's plot. Whatever's up with the crazy kids, their nut of a father, and the whatever-it-is that's murdering people left and right, now's the time to bring it to a halt. Maybe it ends with some terrifying ritual straight out of Cultes, or maybe with one final rant from whichever madman is behind this scheme. Then comes the bloodletting, and since this is Technicolor there ought to be a lot of blood to let: servants, passers-by, policemen, doctors, and so on. Theoretically you could dispense with the fourth scenario and have it all happen in the third, but it may be more satisfying - and less confusing - to let Hardy's final conflagration have its own scenario.
At the end of it all the characters will be battered, bruised, but hopefully unbowed, and have their own copy of the Cultes des Ghoules. They also know a lot about du Bourg's, and have some power of their own, either through the optional rules or because they've been reading Mythos texts.
The question is, what happens next?