It’s not easy to find a simple online explanation of the difference between Official and Unofficial sources of law. My explanation may fill in that gap – or not, depending on your specific question. (And a blog post this long can hardly be called “simple,” but such is life — and law.)
1) In a nutshell, an official source is a source that has been authorized by an official body, such as a court or a legislature. In Oregon, for example, we have the official statutes of Oregon, published in the Oregon Revised Statutes (ORS), by the Oregon Legislature, through Legislative Counsel. (This official statutory compilation should not be confused with Oregon Laws, which is the official session law compilation.)
3) Most legal dictionaries will define the word “official.” I like the definition of “official” in the “Legal Research Dictionary: from Advance Sheets to Pocket Parts, Second Edition,” by Elyse H. Fox. It not only defines “official” in the context of legal research publications, it also includes the point that official publications may be published by a private publisher, under the authority of a government entity. That is, a government entity may delegate that publishing authority to a private publisher, e.g. Thomson-Reuters or Lexis-Matthew Bender.
a) A statute codification can be official, but not positive law. It may be prima facie evidence of the law (rather than “legal evidence of the law.”). For the record, the ORS is an official source of the law, but it is not positive law.
b) For information about drafting legislation, take a look at the Oregon LC Bill Drafting Manual.
5) And then we have Primary and Secondary legal research resources, though that is a different subject altogether because secondary resources are generally not sources of law, but are commentary on the law and law finding tools. Wikipedia does a pretty good job at explaining the difference between primary and secondary.) And the American Association of Law Libraries (AALL), Legal Information to the Public Special Interest Section, has an excellent introduction to legal research.