In both books, Achilles is unforgiving in his anger, one of the themes. He will not relent in either book. The difference is that in Book 9 he will not forgive Agamemnon for taking Briseis, even though Agamemnon is willing to give her back and pay him back handsomely. This shows us that Achilles is stubborn and too proud to forgive Agamemnon.
In Book 24 we see a different side of Achilles. The book begins with him dragging Hector's body behind his chariot over and over again. He wants his revenge, another theme, just like he wanted it against Agamemnon in Book 9, and once again he is stubborn and won't give up Hector's body. The difference then is when Priam appears and begs him for his son back. We see a difference in Achilles here. He gives Priam his son's body. He gives in because he is reminded of his own father when talking with Priam. This is a different Achilles than the one we saw in Book 9. Together the men cry over the loss of humanity altogether, not just for their loved ones. Together they recognize mortality for what it is, which is another theme.
Both books appear to be an end of something big in plot development. Book 9 sounds like it might be the end of fighting and war for the Achaians, as they may be heading back home the next morning. The last book ends rather abruptly with the funeral for Hector. The fighting is no more, and Achilles, the important main character of the epic, has transformed.