Alan Garner’s books, Elidor, The Owl Service, The Wierdstone of Brisingamen and The Moon Of Gomrath are classics, The Owl Service would be too dark for a child of her age, it’s dark for an adult reader!
Catherine Webb wrote a bunch of YA books, her Horatio Lyle books might be of interest, they’re a sort of steampunk Sherlock Holmes/mad inventor, the main character is a former Special Constable with a passion for science and invention who does some sleuthing, with help of a group of children. The first one of four is The Extraordinary and Unusual Adventures of Horatio Lyle.
Her other YA books, Mirror Dreams and Mirror Wakes, and Waywalkers and Timekeepers are more for older readers, the first of those, Mirror Dreams, she wrote aged 14, but it reads like a Roger Zelazney novel.