Because restatements are one of the most influential types of secondary authority, you might want to consult the Restatement of the Law, Third: The Law Governing Lawyers. A few notes:
- The coverage in the restatement is broader than state ethics codes (or the model rules).
- In describing the law, it looks not only at jurisdictions' ethics codes but also "decisional law," meaning the Restatement may differ from the official text of the model rules. Moreover, at times the Restatement authors advocate what the law should be. The foreword notes, "These departures [from the code formulations] are carefully considered and were extensively debated. As those of us involved in the drafting of the codes will testify, many of these departures simply clarify the intendment of the code provisions and others seek to supersede drafting mistakes. Other departures reflect recognition that experience with the codes revealed that better resolutions were to be had on a variety of issues." Restatement (Third) of the Law Governing Lawyers Foreword (2000).
- There is no first or second edition of this title.
Restatement of the Law, Third: The Law Governing Lawyers by 
