Pip receives another letter. What does it say and why does Pip feel obligated to leave town immediately?

Again I'm not sure what chapter you are on. In chapter 52 Pip receives a letter from an unknown person. They want to meet him in the marshes at a particular site in his home town. They have information about Magwitch. He is supposed to come alone and bring the letter. Pip feels obliged to help his own benefactor. Pip also receives other letters like from Estella.