I was given a 2 in 1 book: 'What Katy did at school & What Katy did next''. These books are the 2nd and 3rd in a trilogy. But I was thinking I wouldn't need to read the first (What Katy Did). Also, is this book a classic? Do I really need to read the first.
They're by Susan Coolidge.
They're by Susan Coolidge.

I read them when I was a child, at least the first one and loved it, I'd say Yes!, by all means read the first one, it was crucial to Katy's development as a person and her character.