To one as young as you, I'm sure it seems incredible, but to Nicolas and Perenelle, it really is like going to bed after a very, very long day. After all, to the well-organized mind, death is but the next great adventure. You know, the Stone was really not such a wonderful thing. As much money and life as you could want! The two things most human beings would choose above all - the trouble is, humans do have a knack of choosing precisely those things that are worst for them.
I guess the second it indicates to die or death, and the first it indicates "Nicolas and Perenelle ~ adventure." I guess "those things" has no its match in the text, but it implies the worst concrete choices one makes in his life. Are these right?