It's probably pretty clear to you why Manzanar is important in Farewell to Manzanar, so we're not going to dwell on that point.
Farewell on the other hand is good for a small pause. Think of farewell as an act that's all about letting go of trauma and feelings of crisis, so it makes sense that the whole book is about a farewell to Manzanar. After all, Manzanar is the place that represents and forms Jeanne's biggest fears—of being Japanese, of racism, of leaving a small world in favor of the world outside camp.
Saying good-bye to such a place is no easy thing, which is why it takes a whole book to accomplish—and make sense of—the title.