Lots already mentioned, particularly The Borrowers, The Family from One End Street, Mallory Towers and the Little Women and Heidi series, but I’ve been trying to think of some of my favourites that haven’t cropped up. There was a whole series of books titled The Young……. (Marie Curie, Elizabeth, Florence Nightingale, etc, etc), which I loved. Famous men were included as well, though I didn’t enjoy their stories nearly as much - I was always scouring the library shelves for a new heroine to read about. I also loved The Bobbsey Twins, though I can’t have paid much attention as I never even cottoned on to the fact that they were American.
I think I was hooked by the idea of a family with two sets of twins, always off on adventures. Another favourite for some reason was Tom Brown’s Schooldays.