It is a bit too inside baseball for me to fully enjoy, but Beyond the Formalist-Realist Divide: The Role of Politics in Judging by Prof. Tamanaha* is recommended. He discusses the book here. A taste:
I call this “balanced realism” because it acknowledges the limitations inherent to law and human judges—which cannot be eliminated—yet it also recognizes that law nonetheless works, that judges can and do render rule bound decisions. For at least two centuries, the book shows, judges and jurists have described law and judging in balanced realist terms. The formalist-realist divide that dominates contemporary views of judging tends to obscure this common ground.
The need for judgment, one that will open up some disagreement, is relevant here because he still believes that the "rule of law" is both possible and fundamental. This requires some minimum standards, and he cites research to show that judges as a whole do agree in most cases.
frequently there are legally “correct answers” and “incorrect answers,” as determined by prevailing legal conventions and practices
Thus, though some don't agree it has been violated, there is some minimum standard here that someone in Yoo/Bybee's place could have violated. I fear from reading some of the discussions that some disagree. That in effect they had a job to spin the law to meet the needs of the administration and to challenge them on that is the outrageous thing. As the author notes, the assumption that the law is merely political and partisan is likely to promote just that.
The book has received lots of kudos from the legal community, including from the likes of Prof. Kerr over at Volokh Conspiracy, the opposite number blog of the author. This helps me respect it, since clearly I'm not able to fully judge the research behind it with my more limited knowledge of the subject. I do think someone shouldn't take away from the book that those terms are meaningless. But, as with "originalism," there can be a lot of exaggeration.
This article of a "formalist" that admits to some discretion in close and novel cases, cited in the book, is also a good read.
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* He at times blogs at Balkinization, providing sound commentary with a polite style, a good balance to some of the excitable [Sandy Levinson honestly feels our political system is in deep **** at the moment, but really needs to calm down sometimes. Still, everyone needs a place to vent.] or somewhat elitist sounding others there. This includes those who don't allow comments.