[Back after a year long hiatus with a couple new players and a resolve not to let this story die in the water.]
The group gets a tip from a previous captive about where the cult can be found and where they keep the book. They scout the warehouse in question quite thoroughly, sneak in masterfully, then squander all that nice progress by confronting the High Priests (in disguise) openly. A fight ensues that results in the Book of Gates being recovered. The group makes a strategic withdrawal and heads for their apartment… because there’s no chance the bad guys will look there.
(If I seem a little derogatory toward my party, it’s because I am.)