I bumped into these books because of the the nice cover designs, when volunteering at our elementary's library. Turns out they are set in the Bohemia of an alternate Europe, with Czech speakers and all that. A little steampunk, a little magic, but mainly a great, spunky main character, Petra Kronos, with terrible, terrible problems.
I found the first book enchanting, the second gripping, the third heartbreaking, and I liked how the author lined everything up so that a young teen could, with all the allies she gathers, have a shot at saving the world.
I also enjoyed how the books' villain was motivated; he was simply single-minded in his pursuit of power, and that was more than enough.
But probably the reason I actually sat down and read it was the scholarly and helpful tin spider, Astrophil, who is the main character's pet. He offers sage advice and reads at night while she sleeps.
Great stuff. It's marketed as youth or young adult, but I thought it played well as an adult book series.