In Cogman's well-constructed environment, there are actually multiple worlds (and alternate worlds) in existence, most of which have some level of magical powers present. (Think a sort-of-Steampunk-meets-magic place, as Irene's current base of operations is a Victorian London.) The fae and other magical folk, not-too surprisingly, can move between these worlds; regular humans can only do so with a little magical help. And then, there are the Librarians--normal-ish people who study, intern, and are eventually hired by great institutions known as Libraries--who can move between the worlds (along with being able to do a host of other useful things, when needed) by their use of a special Language (and generally, a proximity to books, if not an actual library). The Librarians' main purpose is to--ehem, acquire--rare books for the particular Library where each of them is employed... by whatever means necessary (and yes, you should draw your own conclusions from there).
In this outing, Irene isn't tasked with finding a book, but rather, takes off on her own when her apprentice--the (only-slightly-younger-than-herself) dragon, Kai--is abducted by a nefarious husband-&-wife fae duo and transported to another world (an alt-version of Victorian-era Venice, as it happens)... one in which his own powers (not to mention, that of his family) are very weak.
Cogman's descriptions--particularly as seen through her protagonist, Irene's, eyes--are sumptuous... but therein lies part of the problem, for me; the author tends to go on, a bit, leaving me to skim long passages while searching for the next thing (as in anything) to happen. Some of the scenes with multiple characters are much the same; after I, as the reader, had "gotten the point" in a scene, it would have behooved Cogman to pick up the pace, again, rather than belaboring said point(s). In other words, another pass of editing may well have tightened things up just enough to omit the definite lags I experienced. (Visuals can only carry one so far, before one wearies of a sight... no matter how breathtaking or fascinating that sight might be.)
Still--provided you're willing to put yourself through a bit of wading-&-skimming--I'm give this book a strong recommendation, on the basis that it's a competent continuation of an interesting, compelling series which sets itself apart from other Steampunk-ish/historical-urban-fantasy tales by sheer dint of originality.