I don't know if this just happens to be a thing that's common in the books I read as a kid, but there was always that bit, in the beginning of every book in the series, where the main character(s) were described. It was pretty much the same description in every book (the Nancy Drew books and the Sweet Valley Twins books come to mind here). In these books I sort of get it, cause you pretty much always picked the book that looked most interesting in the library and therefor never really read them in the right order.
Now last night I had to look up a thing in the third Harry Potter book and I realized that the same thing goes on here. Why would you in a book series that is meant to be read in chronological order describe the main character(s) in every book? Wouldn't that just bore the reader? I might be interested in knowing Harry has grown a few inches, but I already know he has dark hair and green eyes.
In what circumstances is it appropriate to describe the character in every book in the series and when should it be avoided?