Privacy rights most often arise if you are seeking to use third party content like correspondence, diaries, and images that contain personal information about or pictures of particular people.
In addition to a number of federal statutes that protect against disclosure of various types of personal information (e.g. FERPA (student information) and HIPAA (health information)), there are also state laws governing privacy.
State privacy laws make intrusions of privacy a tort (i.e. a wrongful act). The four typical types of intrusions that state laws protect against are: intrusion upon seclusion; public disclosure of private facts; painting someone in a false light; appropriation of name or likeness.
You may not need to dig too deep. There are important limitations on privacy rights in the context of your scholarship.
First, privacy rights expire at death--meaning, you can't be liable for disclosing private facts about a person no longer living.
Second, there are typically two other important defenses to claims for invasion of privacy: newsworthiness, and permission. If the material you wish to include reveals private facts that are "newsworthy" (of public interest or concern--which your dissertation scholarship may be), or the person who is the subject of the information has given you permission to publish (which you may have obtained), then an invasion of privacy claim should not be sustainable. For more on newsworthiness and permission exceptions, exploring the Digital Media Law Project out of Harvard University's Berkman Klein Center should prove helpful. It is no longer being updated, but contains very useful explanations.