When reading Supreme Court opinions, I like to keep track of "nuggets," particularly nice statements of basic principles that may be useful in other contexts. The Heller opinion has a number of nuggets on the interpretation of enactments, applicable both to statutes and the Constitution.
"It is always perilous to derive the meaning of an adopted provision from another provision deleted in the drafting process." (slip op. at 16)
"But where the text of a clause itself indicates that it does not have operative effect, such as 'whereas' clauses in federal legislation or the Constitution’s preamble, a court has no license to make it do what it was not designed to do. Or to put the point differently, operative provisions should be given effect as operative provisions, and prologues as prologues." (fn. 5)
"'"It is nothing unusual in acts . . . for the enacting part to go beyond the preamble; the remedy often extends beyond the particular act or mischief which first suggested the necessity of the law."' J. Bishop, Commentaries on Written Laws and Their Interpretation §51, p. 49 (1882) (quoting Rex v. Marks, 3 East, 157, 165 (K. B. 1802))." (slip op. at 4-5)
"'Legislative history,' of course, refers to the pre-enactment statements of those who drafted or voted for a law; it is considered persuasive by some, not because they reflect the general understanding of the disputed terms, but because the legislators who heard or read those statements presumably voted with that understanding. Ibid. 'Postenactment legislative history,' ibid., a deprecatory contradiction in terms, refers to statements of those who drafted or voted for the law that are made after its enactment and hence could have had no effect on the congressional vote." (slip op. at 32)
Legislative history "most certainly does not refer to the examination of a variety of legal and other sources to determine the public understanding of a legal text in the period after its enactment or ratification. That sort of inquiry is a critical tool of constitutional interpretation." (slip op. at 32)