The personal identity literature is fragmented. There is a literature on the normative topic of 'what matters in survival'. And there is a separate literature on the metaphysics of persons. But in Self and Identity, Trenton Merricks shows that some important claims about personal identity cannot even be articulated, much less evaluated, unless these topics are brought together.
Merricks says that what matters in survival is constituted by its being appropriate for a present person to first-personally anticipate, and have self-interested concern with regard to, a future person's experiences. So what matters in survival is not constituted by identity with a future person. So identity is not what matters in survival. But Merricks argues that--given a metaphysics of 'enduring' persons--identity with a future person explains why it is appropriate to first-personally anticipate, and have self-interested concern with regard to, that person's experiences. So identity delivers what matters in survival.
Some claim that what matters in survival is delivered not by identity, but instead by psychological continuity. Or by having the 'same self' (that is, the same values, desires, and projects). Or by narrative connectedness. Or by unity of agency. Merricks argues that these claims--unlike the claim that identity delivers what matters in survival--cannot accommodate all the ways in which personal transformations can be good, or bad, for someone. At the end of Self and Identity, Merricks puts his conclusions about what matters in survival through their paces by applying them to a new topic: personal immortality.