This National Review article is the most comprehensive but concise statement I've seen so far on the tangled statement of the arguments to be this week in King v. Burwell. It even sorts out the confusing state of the many decisions below, most of which are being held in abeyance pending the Supreme Court's decision in the case that is being argued this week. There is more information than in most articles about the sorry history of the making of the IRS regulation (recall that this lawsuit challenges IRS rules that violate the law; it doesn't challenge the law itself).
The article passes lightly over Congress's intent, as is right in a case where there is no statutory ambiguity, but here is another article that lays out quite clearly how absurd it is to argue that there is any real question that Congress considered versions of the law that did and did not restrict subsidies to states that implemented their own exchanges, and in the end was able to pass only a law that did restrict subsidies in that way. To the extent the IRS rule has a shred of validity, it can only come from the argument that it has discretion to implement rules consistent with a law's "purpose" even if the statutory language is ambiguous. If this language is ambiguous, we might as well throw out all of our laws--which, come to think of it, is pretty much what our current administration is up to.