I know I have droned on about this probably ad nauseum, but yes, Vornheim, The Complete City Kit by Zak S. continues to be the most used game product off my shelf during live play. I do use it sometimes for game prep, but, and I believe this is the point of the book, it works best for me when, midgame, I spin around in my chair and literally grab this small 64 page book off my bookshelf and put it to use in the moment.
My continuous campaign is set in 1646 England so the color, the fluff, of the book dovetails pretty seamlessly with this campaign world, but the nuts-and-bolts of the content, when stripped of the fluff, is a real honest-to-grimness Game Master/Referee tool I can use in any genre I'm running. The book has made a part of gaming I always found difficult to run a breeze and a joy. See, when I'm jamming a session there is a lot of noise going on upstairs in my head; what's around the next corner, what are the bad guys doing right now, how does the player's actions effect what is going on, should I arrest them... ?
It may seem simple when a PC wants to make contact with their secret organization where would it be located, but it is just such tidbits I don't know which can bring my brain to an awkward stop, and worst of all, break the flow of play which most certainly is going on for the PC's.
For example, today I only used the book once, but as I mentioned a PC wanted to make contact with their faction, a secretive organization which may or may not have a chapter in the current city. I called for a Streetwise skill roll for the PC to establish in a roll whether or not there was a chapter here for him to make contact with. If the roll fails then no luck. But the PC nailed it so I need to give him satisfaction. Go to the back of Vornheim, roll on the table and wala - Orphanage. It kind of bakes in what type of faction leader you are going to find. If I rolled a jeweler the location in the city, the contact's personality, all would be kind of different than the type of personality involved with an orphanage. Not saying the contact would be caring of the innocent, perhaps they hate kids with such a passion the orphanage allows them to enact their viciousness with no one noticing, but it does start the world building process right there at the table with credible elements. It is the English Civil War, there are going to be many orphaned and displaced children, who knows what they have seen... hooks just start mushrooming and all I have to do is sit back and wait for the PC to act. There is much more to the book than just building names, any of the numerous reviews out there can give you tons of detail. The big takeaway for you the reader is it is an in-play game tool which works!
This book is totally worth its weight in gold - highly recommended.