The Meditations is basically Marcus's personal journal. It's a record of thoughts that is carefully composed, but it was never intended for a wider audience. Originally, it wasn't organized into the books, chapters, and sections we see today when we open any edition of the work. In fact, it can be hard to locate particular discussions within the Meditations, since they aren't necessarily grouped thematically.
While all the books in the Meditations discuss existence, mortality, virtues, relationships with community and the gods, reason, duty, and death, certain books in the work stand out for their strong thematic concerns. Book 1 is an extended shout-out to the people who shaped Marcus in some way, while Book 3 lays down some of Marcus's foundational ideas, taken from his favorite Stoic philosophers.
Book 6 addresses duty, among other things, and Book 8 represents a time of reflection and regret in Marcus's life, when he realizes he will never be a real philosopher. Book 9 gets pretty theological. In this one, Marcus discusses sin—both against the gods and humanity—and how man must behave in social ways for the good of the universe and himself.
Book 10 has a religious feel to it, with Marcus's sensibility that "all is well and all shall be well for you" revealing his supreme confidence in the benevolence of the gods that order the universe.
By the time we reach Book 12, Marcus is nearing the end of his life, and he's extremely interested in summing up and reiterating for himself his most important principles. He's not sentimental or nostalgic at all here, but he is concerned about being in a good place for death.
Marcus ends with an envoy—a kind of farewell P.S.—that speaks of his readiness to receive his marching orders from the world, confident that the gods are his friends.