The Red Record by Ida B. Wells-Barnet deals mainly with statistics on lynching. How do I identfy the protagonist in this book?

Is there some reason that you believe you ought to be able to find a protagonist in this book?  In general, you would not say that a non-fiction work (especially one that is mainly statistical as opposed to, say, a biography) has a protagonist.

In a work of fiction, the protagonist is the hero -- the person that the reader or audience is supposed to identify with.  Is there a person in this work that you are meant to sympathize with?  That's how I would look for a protagonist.

I suppose you could say that blacks in general are the protagonist.  Or that the individuals whose stories are told are the protagonists.  But I fail to see how you could identify an individual who is the protagonist for the whole book.