Some of these might be a bit old fashioned as it’s based on what me and DH read as avid readers at that age (we have every book DH has ever owned in our house currently so am using the bookshelves for guidance), but also what we’ve bought for our nieces / nephews :
The Wolves Chronicles by Joan Aiken
Skellig by David Almond
Noughts and Crosses series by Malorie Blackman
The Molly Moon series by Georgia Byng
The Underland Chronicles by Suzanne Collins
The Dark is Rising Sequence by Susan Cooper
The How to Train your dragon series by Cressida Cowell
Love That Dog by Sharon Creech
The Last Dragon Chronicles by Chris d’Lacey
Books of Ember series by Jeanne DuPrau
The Neverending Story by Michael Ende
The Michael Vey series by Richard Paul Evans
Flour Babies by Anne Fine
Harriet The Spy by Louise Fitzhugh
The Ranger’s Apprentice series by John Flanagan
The Inkheart series by Cornelia Funke
The Surya trilogy by Jamila Gavin
Paul Gallico books - Jennie, Thomasina the Cat Who Thought She Was God, Manxmouse
The Spy School series by Stuart Gibbs
The Once series by Morris Gleitzman
Journey to the River Sea by Eva Ibbotson
The Redwall series by Brian Jacques
The Nancy Drew series by Carolyn Keene
The Roman Mysteries by Caroline Lawrence
The Earthsea series by Ursula Le Guin - especially Wizard of Earthsea (the first book)
A Wrinkle in Time by Madeleine L’Engle
The Giver Quartet series by Lois Lowry
The Furthermore series by Tahereh Mafi
The Greenglass House series by Kate Milford
Michael Morpurgo books - Kensuke’s Kingdom, The Butterfly Lion, Private Peaceful, Why the Whales Came, An Eagle in the Snow etc.
Echo by Pam Munez Ryan
The Rats of Nimh series by Robert O’Brien and Jane Leslie Conly
The Inheritance Cycle series by Christopher Paolini
Brian’s Saga series by Gary Paulsen
The Chronicles of Ancient Darkness series by Michelle Paver
The Adventure Series by Willard Price
The York series by Laura Ruby
The Silver Sword by Ian Serraillier
A Series of Unfortunate Events by Lemony Snicket
Treasure Island by Robert Louis Stevenson
The Goosebump series by RL Stine