This page notes some key resources for the ethical rule revision assignment. A more complete resource list is available on the legal ethics research guide:
If you were researching in a specific state, you would start with that jurisdiction's rules of professional conduct, preferably in an annotated version. You might look at secondary sources specific to the state (such as the Iowa Practice volume, Lawyer and Judicial Ethics), as well as state-specific disciplinary and ethics opinions and case law.
The American Bar Association's Model Rules of Professional Conduct informs the ethical rules in all states. California revised its rules to more closely align with the model rules, and all other states have adopted some version of the model rules. Therefore, you might look at the corresponding rule in the model rules, the development of that rule, and analysis of that rule's meaning. If there was not on-point case law or ethical/disciplinary opinions in your state, you might look to another jurisdiction to see if they had relevant decisions you could use as persuasive authority.
For more in-depth background on the development of the rules (which might provide insight into their intent), see the most recent ABA Center for Professional Responsibility legislative history of the model rules:
In addition to sources organized by rule number, a large number of sources address aspects of legal ethics topically. Some specific titles are on the Legal Ethics research guide. 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:
You could also try narrowing a broad SuperSearch search for the subject of legal ethics, using the advanced search feature to add terms on your particular area of interest. Here is an example related to email communications.