Plagiarism is the deliberate and knowing presentation of another person's original ideas or creative expressions as one's own. Copyright infringement is the use of another person's original and creative expressions as one's own that violates the copyright owner's exercise of rights in the work. Plagiarism and copyright infringement are related, although if one is guilty of plagiarism they would be sued for infringement and not for plagiarism.
Plagiarism is the act of copying and presenting another's work. An infringement suit is the potential result of the act of copying and presenting of this work.
The facts of the family history as displayed in the emails, in the form they are presented in the emails (their structure, interpretation, characterizations, tone, etc), could have enough original and creative expression to be protected by the copyright law, so an exact reproduction of the emails as written could potentially be seen as infringement (and there have been cases where the exact reproduction of letters by the recipients of the letters has led to infringement actions initiated by the authors of the letters). The facts (dates, times, locations) contained in the emails or letters, on the other hand, can generally be pulled from the emails and used by anyone.
Facts (and ideas), as divgradcurl and The Occulist have said, are not copyrightable.
There could potentially be a violation of rights other than copyrights, with the use and publication of family interview information. For example, if an interview covered personal or private matters, and the information revealed during the interview is used without permission from either the interviewer or the interviewee, there could be problems. The publication of this information could potentially lead to an invasion of privacy action, depending on what exactly was revealed, why, and under what conditions. Problems could arise if the information is later published contrary to the conditions originally set out for the interview. And the facts themselves, should they be questionable facts in any way, could potentially lead to other actions (ie. defamation).
Without these additional factors to complicate matters, though, the factual information you shared with the other person is able to be used by this other person in her book, whether the book is for profit or not. The copyright in the photos, as mentioned by divgradcurl, would generally belong to the photographer who took the photos, absent any agreement to the contrary and absent them being in the public domain. Without permission from the photographer, the use of the photographs in any book could lead to a copyright infringement suit.
If you believe your emailed material is being used in a way that violates any underlying rights you (or anyone else) may have in the material, your best course of action is to have the emails and the published book reviewed by an attorney in your area, who can determine this better.
