As sunlizzard says, it's an old joke. As I recall, it involves a tourist wandering the streets of New York City, badly lost. Finally, in desperation, he stops a local and asks, "How do I get to Carnegie Hall?" The local looks him up and down and replies, "Practice, practice, practice!"

[Actually, I have a vague memory that a cabbie's supposed to be involved in the joke, but I can't make it work that way.]

Honestly? I don't know how; I don't recall it ever being a problem, at least when I'm actually reading them. After time passes, I may get confused as to what happened in which book, but not while I'm reading them.