I'm certainly not saying you should need one book to use the other. Do you need the Lovecraft country sourcebooks (Arkham, Kingsport, Dunwich, Innsmouth) to use More Adventures in Arkham Country or New Tales of the Miskatonic Valley? Instead, they're designed so you have everything you need, but if you have the sourcebooks as well you can use them to flesh the scenarios out. Information is only duplicated if it's directly relevant to a scenario.
Ooh ouch. Tricky territory here. Do we want to say you can't use this book unless you get Secrets too?
You should be able to use one without having the other.
This sounds eminently sensible! If there's info you feel needs to be included to make the scenarios work properly, it should of course be included. But I'd be annoyed at paying for a book and finding it duplicates a lot of stuff I already paid for.
Well, this is a collection of scenarios, not a source book. Tom and I felt it would be helpful to include some source material for gaming in NYC, however there is little point in repeating what is already in Secrets of New York. No, you will not need one book to fully utilize the other: the one will simply compliment the other.
That would be cool. I don't know if it's possible, but I'd certainly like to see such info. I think CoC books have been pretty lacking in info on organised crime generally. Though as Scott points out, this isn't a sourcebook, and I wouldn't want it to get bogged down in such things.
It might be worth bearing in mind that the German version of Secrets of New York adds a chapter on organised crime, as this is an element of life in 1920s New York that the Germans apparently thought was neglected in the Chaosium version. Maybe you should address this perceived shortcoming in your book?