I read The King of Attolia and A Conspiracy of Kings one right after the other, after having taken a break after the second book in the series, The Queen of Attolia. I am glad I took a break, but also thrilled I had books 3 & 4 available without having to wait. Again, Turner trusts to the nimble-mindedness of her reader by crafting a tale of political intrigue and power plays. I loved all four books. I have heard rumors that she is not done with the series, but A Conspiracy of Kings ended so tightly I am not sure there are any threads to play with to write a 5th book.
While these books are intricately plotted, the reading level and the lack of anything "inappropriate" would make these good reading for advanced children, maybe grades 4-6. They are certainly aimed at young-young adults, grades 7-9. I would also call these high interest for the high school level or "true" young adults.
I am not sure what I like most about the series, the characters, who are interesting with depth and all the quirks and inconsistencies that make people fascinating and infuriating, or the plot which has completely unpredictable but believable twists and surprises. Whichever it is, Turner has crafted a brilliant series that I could seriously read again...if I had time, ha!