The Diamond Conspiracy - Pip Ballantine, Tee Morris, Philippa Ballantine
Most authors let a series go for awhile before they break formula, but not these two -- here were are  in Book #4, and they're shaking things up in this follow-up to that intriguing cliffhanger.  On the one hand, I wish they'd given us a one or two more of the standard Brooks & Braun head off on a mission somewhere (even to another country or something like their trip to the States) before this one -- but I'm not sure it could've waited.  
You get a glimpse of what's coming in the previous book -- the Ministry has been decommissioned.  The agents who served the Crown so faithfully are now hunted by her government.  Yet, they stay true to their mission, as you'd expect.  Not the easiest task, even for these agents, used to the peculiar and the impossible. And what they're about to go up against is a lot more peculiar than they're used to. I'm always a sucker for tales of second chances -- and we got a few here, to boot.  This book really has a little bit of everything.

Beyond the events of the story, there is just so much here to blow the reader's mind -- events, revelations, characters -- in the last hundred pages that I cannot talk about without utterly ruining the experience for anyone. But if you don't sit up in your chairs a little straighter, mouth agape, at least twice in Chapters 14 and 15 (for example), there's something wrong with you.  And the last chapter?  Great, just great.  
I've always enjoyed the back and forth between the Brooks and Braun, but now they've added this flirtatious (and then some) aspect to it, making the scenes between two cackle with a new energy.  The growth in their relationship feels natural, and doesn't go too far.  Sure, they're ga-ga over each other, and in the first blush of new love -- but they're still professionals, with work to be done.  Now that there's a little more trust in each other, maybe that works better, but it's still the same base relationship we've been following since Phoenix Rising.  
One final thought: Is it just me or does that position Eliza is in on the cover look: 1. really uncomfortable, and 2. not that useful for shooting?
A dash of steampunk, a bit of romance, some intrigue, some "you've got to be kidding me" moments, and good character progression.  This is one of my favorite series going, and this entry just solidifies it.