Legal Question in Constitutional Law in California

Re: Can I publish letters written to me?

Generally no, but the answer depends upon where and how you plan to publish it, and also upon facts about the original letter. You haven't provided these facts, so I can't give you a more definitive answer.

Buying a book does not give you the right to publish it, and neither does buying the original manuscript. The same is true of letters and other written materials. The physical letter itself -- by which I mean the paper and the ink -- is yours to keep, lend, sell or give away as you please, but the content is not.

Of course, whether the author will care enough about your actions to sue you is another matter.