Agatha H. and the Airship City (the novel) is a second look at the Agatha Heterodyne "Girl Genius" story now being developed in the authors' graphic novels, the most recent two of which have won the Hugo for Best Graphic Story. As such, it should be taken as a secondary source.
This volume covers the story we met in the first three volumes of the graphic story: Agatha Heterodyne and the Beetleburg Clank, Agatha Heterodyne and the Airship City, and Agatha Heterodyne and the Monster Engine.
Phil Foglio has stated in interviews that one part of the Girl Genius story is what happens to legends over time. The legends about Agatha's parents and uncle have grown with time; should we not expect the same of the legends about Agatha? In fact, there are some details changed and snippets of story added. A prologue tantalizes with glimpses of history and historical characters and perturbs the historical timeline developed by fans, hinting at clues or vital constraints. Other changes deepen and richen the character development without, so far as I can tell, damaging the story. We learn a few new things and are reminded of some old ones. Some lines of fan speculation are cut off, leaving us to focus on better questions.
The Girl Genius story is all about the backstory. Bit by bit through the graphic novels we've gotten history, found questions in it, and gotten answers that have provided fascinating and bigger questions. That essential story dimension does not change.
The Foglio humor still glows. Much of it revolves around the Jaegermonsters. If you are a fan, you probably know that the Jaegermonsters were a late addition; Phil was already drawing the comic books in which the first part of the story appeared when they were invented. By now they are deeply woven through both story and backstory. Among their narrative duties, they serve as clowns whose antics conceal both foreshadowing and backstory. A few new incidents near the start of the novel had me laughing out loud. (Clowns or not, they are capable of noble action and sacrifice.)
Is this as good a novel as the graphic story is a graphic story? No, it is not. That's not bad news: the graphic story's artwork, pacing, and working out of story details are so good that it would be a wonder if the novel could match it. The most apparent weakness is that details of the milieu that can be background in graphic form must be explained, and sometimes the explanation pauses the story for detail that might better be given elsewhere. This is a matter of technique and a basic problem for all SF&F writing, and unless the Foglios choose not to improve (for the sake of style, perhaps) they surely will.
It is possible that writing this novel required the Foglios to make another pass over their story notes. The overall plot and the key characters were written before the first published page was drawn, promising a tightly woven story. Phil and Kaja Foglio have delivered magnificently on that promise. This novel may help them continue to do so.
If you want the primary source for the Girl Genius story, continue to buy the graphic novels and read the eponymous web site. That's what they are for. If you are already a fan of the story, or cannot bear the long graphic form, then buy this book. But if you skip the graphic novels, you are missing an awful lot of richness, depth, and fun.