Heidi by Johanna Spyri, sequels by Charles Tritten. I read the original Heidi when I was little & loved the beautiful storyline - I then read the sequels (written by another author) when I had found them out as a teenager & thought they did justice to the original. Definitely recommend these books to children & teenagers.
Joanna SPYRI (1827–1901) -- She was a Swiss author of children's stories, and is best known for her book Heidi. As a child she spent several summers in the area around Chur in Graubünden, the setting she later would use in her novels.