Getting Permission. But, assuming the permission is forthcoming, it occurs to me that publishing her letter, even with a copyright clearance, also invades her privacy. Do I also need a release for this, or is the copyright permission alone sufficient to protect me? Ah, love letters ... Do people still put pen to paper any more? It's hard to believe that the mailbox was once a place where you could find intimate correspondence. We are reminded of the words of W.H. Auden: "And none will hear the postman's knock/Without a quickening of the heart./For who can bear to feel himself forgotten?"
Right, you had a question. Your copyright permission -- assuming it mentions the intended use in your memoir -- should protect you from claims of invasion of privacy. Invasion of privacy is typically either the public disclosure of private facts or it's an unwarranted intrusion into someone's private affairs. So, if your former girlfriend is consenting to publication, it would be difficult for her to later claim that her privacy is being invaded. There is one possible exception to her implied release of claims, and that is if you use false information -- for example, you deliberately misquote the letters -- in order to place your girlfriend in a bad light. In that case, she may have a claim for "false light" invasion of privacy or for defamation. In any case, it would be hard to shield yourself from these latter claims. We imagine that a former girlfriend would be hesitant to sign a bullet-proof "I won't sue about anything" release unless she had the ability to review the memoir ahead of publication.