This is the written novelization of the first two volumes of the Girl Genius graphic novels. In other words, if you are already a fan, there is nothing new here, at least not technically. But I still recommend reading it, and not just from a “completist” perspective.
There are things that can be done in a traditional novel that can not be accomplished in a graphic novel (and vice versa), including some deeper character introspection.
This was a fun, fast read, and I hope that Phil & Kaja Foglio are able to do more novelizations of the Girl Genius stories.