http://www.girlgeniusonline.com/comic.php. The archives are available online as well as in 11 volumes (and counting) of dead tree editions. After their first few Hugo Awards they started working on novelizations of their story. The first book, Agatha H and the Airship City, covered the first three volumes of the graphic novel. I just finished reading the second book, Agatha H and the Clockwork Princess, which starts with volume 4 and runs through volume 6.
I can't tell you how much I enjoyed the novel. Phil and Kaja Foglio are great writers with a talent for comedic banter and a turn of phrase that reminds me of Douglas Adams. The book runs a bit long (around 430 pages), but it's well worth it. Even if you're already a fan of the comics, this book adds much to the story that the Foglios couldn't get illustrated.
The story: Agatha Clay lives in an alternate steampunkish version of Europe. In this world there are people known as sparks. They have a particular brand of genius that makes them into mad scientists.
Europa is currently ruled by Baron Wulfenbach who rules from his massive zeppelin. But it wasn't always so. Once upon a time the Heterodyne family of sparks terrorized Europe. But the Heterodyne Boys turned out to be heroes of legend with the future Baron as a friend and fellow adventurer. Then their castle was destroyed, the Baron sent to a distant land (he got back), the Boys missing, the wife of one of them kidnapped, and, so rumor has it, a lost infant daughter somewhere. But that was years ago.
In the first novel Agatha Clay found out that she's the lost Heterodyne heir and that her mother wasn't kidnapped, but was in fact the monsterous spark known as The Other who destroyed Heterodyne Castle and tried to enslave Europe. After finding out she's a repressed spark, she found herself aboard Wulfenbach Castle as a captive and potential wife to the Baron's son. She escaped on a smaller zeppelin with a talking cat and a slow leak. The discovery of this bullet shaped hole is where book two begins.
In this book Agatha Heterodyne crashes in the vast wilds of Europa where she finds a traveling circus/Heterodyne show to hook up with. But as the circus passes through Balen's Gap they're commanded to put on a show. Agatha's convincing performance of her mother gets a prince loyal to The Other to grab her with the intent of imprinting the mind of the mother on top of that of the daughter.
Much of the time Agatha is protected by creatures known as the Jagermonsters. They're humans who have been turned into... well, the best description I can come up with off the top of my head is Klingons mixed with Minions (think Despicable Me). They love to fight and they're loyal to the Heterodynes whether they're crazy despots, heroes, or confused girls in their late teens. And they have some of the best dialog ever.
Hopefully, there will be a third novel. The first two were through Night Shade Press which had some financial difficulties. Other publishing companies have offered to buy Night Shade's properties so Night Shade can pay off their debts, but only if a set number of their properties agree to the new, and rather onerous, terms set by these new publishers. And if they don't agree the rights to their properties may be tied up forever. Check out Phil's post about the deal.
So, go, read, and then buy the graphic novels and novel novels.