Libel is the harming of the reputation of another by making a false statement in a fixed medium (often in writing, but also in pictures, broadcasts, signs) to a third person. Slander is its spoken equivalent.
Truth is a defense to defamation claims but not to invasion of privacy actions.
I suggest you have your book reviewed by a lawyer well-versed in publishing law prior to publication, to avoid your greatest risks of attracting a lawsuit.
There are several threads, some rather recent, that have discussed the legal problems one can encounter when publishing a memoir. You can use the search feature at the top of the page and enter into the "key word" box the word "memoirs." Reading through these will provide you with additional information on writing and publishing memoirs.
Good luck.