The essential problem with the book is that it lacks a suitable villain: a plague bacillus simply will not do, and Dr Vance - while presented as Clare's nemesis - is as much a friend as a foe, here. But I get ahead of myself.
Book One of the Bannon & Clare series was a steampunk Victorian romp from start to finish, gradually building up the "alt-history" component so that what began as the standard foggy London of Sherlock Holmes (or at least, just a few years before that) had by the end of the book morphed into a fantastic world of polished brass clockwork, deadly magic, and science that never was. Building on that would have been a challenge indeed, and I cannot but feel that the author just squibbed it.
Emma Bannon is a Sorceress Prime, and Archibald Clare a Mentath: each is a unique individual, for all that Ms Bannon is the hero and Mr Clare the sidekick. Their relationship is slowly developing, set as it is a few years after Vol 1, but the book could have been set mere days later for all the change that really appears to have gone on. Our team is still great fun to read about, constantly sparring with each other, mostly verbally, but the adventure they get this around, as I said, lacks a villain. Instead, the book appears to merely be setting up Ms Bannon's eventual disillusionment with the Crown she has sworn herself to serve, with the entire plot of the book supporting this need.
I mean, really, yet another twist of the hero questioning their motives and whether or not they have wasted their life? And even worse, an entire book just to set it up? Its not quite as bad as Naomi Novik's Tongues of Serpents for very little really happening, but it seems this is very much a middle book in an open-ended series. It's a shame: Iron Wyrm was great fun from start to finish, whereas this is largely scene setting, for all there are a few evocative moments here and there.