Wow, there are so many. In addition to the many wonderful books people have already mentioned:
The Wolves of Willoughby Chase, Joan Aiken (start of a series, many of them are wonderful, later books are a bit weaker)
Mistress Masham's Repose, T.H. White
the Ramona books, Beverly Cleary
The Clarice Bean books, Lauren Child
Harriet the Spy, Louise Fitzhugh
The Egypt Game, The Velvet Room, and others, Zilpha Keatly Snyder
The Moor Child, Eloise Jarvis McGraw
Caddie Woodlawn, Carol Ryrie Brink
Calico Captive, Elizabeth George Speare
The Worst Witch series (the Worst Witch is a bit of a screw-up, but the books are fun)
Nancy Drew!
The Boyhood of Grace Jones, Jane Langton (OOP, may be hard to find, but I LOVED this book as a kid!)
A Proud Taste for Scarlet and Miniver, E.L. Konigsburg
Daughter of the Mountains, Louise Rankin
Julie of the Wolves, Jean Craighead George
Believe it or not, the American Girls book series are quite well-written and do not try to sell you the dolls. My daughter has especially enjoyed the Felicity (Colonial) and Samantha (Victorian) books. there are tie-in mystery series for slightly older readers, with longer books and more advanced vocabulary, featuring the girls, and she loves those.