George Hillocks analyzes the writing assessments in five different states: Illinois, Texas, New York, Oregon, and Kentucky. For each state, he examines the actual writing test--requirements, prompts, rubrics, and example papers--then he interviews teachers and administrators to get their responses to the tests. It's actually like science!
This is far and away the most rigorous education book I've read. It reads like science, which makes it a bit slower, but there's significantly less rhetoric and emotion than I usually see. It's a very narrow focus: the quality of writing assessments and their effects on teaching in the five states examined.
Some interesting parts: Illinois and Texas had exams that were very formulaic, and that was reflected in the teaching. Kids in high schools were taught the five paragraph essay at the expense of all other kinds of writing. Freshmen English professors at Illinois state colleges bemoaned the fact. New York and Oregon had more broad requirements for kinds of writing, which allowed students to write more detailed and realistic assignments. In Kentucky, the writing assessments aren't a standardized test; instead, the students create a portfolio of works from their classes, English and otherwise. While it's probably harder to grade, this kind of assessment is much more helpful for students--they get to use writing with practical applications, and their teachers aren't forced to focus on standard formulae for writing assignments.