Don’t forget the Jean Craighead Geoge books - the Julie of the Wolves series and My Side of the Mountain series.
She’s also written nonfiction books for children about various animals (Moon of the Mountain Lions, etc) and ecosystems (One Day in the Woods, etc.)
From my son’s generation of books, Eva Ibbotson, and Dianna Wynne Jones come to mind, I loved the humor in these books, matches my own. For more gruesome humor, find “The Children’s book of Cautionary Tales” , which reputedly is/was a victorian era book that Edwin Gorey illustrated in recent times…has wonderful things, like the girl who insisted on walking off all the time and was eaten by a lion at the zoo, or the boy who was always throwing tantrums and ended up as the governor of New South Wales in Australia…
My childhood favorites were EB white (I must have read Charlotte’s web a million times), Roald Dahl, I too loved the “My Side of the Mountain” books, I liked Robert McCloskey as well. Robert Heinlein wrote some science fiction books for young readers that I liked as well.
Space Cadet 
IMO Joan Aiken’s best work is the four books about Arabel (a little girl) and Mortimer (her trouble-making pet raven). Hilarious. Best read aloud in a deadpan voice. The four books in the series are chapter books with wonderful illustrations by Quentin Blake.
The Chronicles of Prydain by Lloyd Alexander
Thank You Mr. Falker or Tikvah Means Hope by Patricia Polacco
And my daughters really loved Shirley Hughes picture books. My favorite was Out and About. I can still recite some of the poems.
The Wheel on the School by Meindert DeJong.
Kate DiCamillio’s The Tale of Despereaux is one of my all time favorites. It is wonderful to read aloud snuggled in bed with your little one or grand child.
Oh Shirley Hughes! I’d completely forgotten about her. I loved her disheveled Moms and messy houses.
I own all of Eric Carle’s books. Love the art. The museum at Hampshire College is a hidden gem.
Agree with above poster…Phantom Tollbooth is a great book!
We spent a great couple of hours in the picture book museum at Hampshire, @thumper1. (The book library is organized by illustrator, so you can pull out and enjoy every book a favorite illustrator drew. Many surprises, and a great way to break up a college visit trip’s intensity.)
Help! I’d love to pick up another couple of books for my 9-year-old nephew who is a good 3rd grade reader. I grabbed a copy of Snow Treasure. What else?
@arabarab, it’s hard to say without knowing what he’s read. The Redwall books are right in that age bracket for many good readers. Along the lines of Snow Treasure, there is also The Silver Sword by Ian Serraillier.
I loved these as a kid–Tunnel in the Sky, Citizen of the Galaxy, Have Spacesuit Will Travel, etc. One note, though–some of his adult books, especially those written late in his life, are definitely not for kids.
Arabrab, I’d second the Redwall books for a third-grade boy who is a good reader. They were a huge hit with my son at that age. (And they have some of the best food descriptions I’ve ever read…enough that 20 years later I still remember thinking that I’d like to be a mythical forest creature just so I could join in the feasting.)
Yeah if you can get the kid started on Redwall - they are in for many happy hours of reading long series of nice fat books! Warning a lot of characters talk in dialect which drove me crazy, but not my kid. The Lloyd Alexander Prydain chronicles would be a good choice - they are written a little younger than many fantasies. I also highly, highly, highly recommend the Swallow and Amazon books - they are about kids sailing, camping and exploring in England. Set in the 1930s or so, though not obviously so. They are so much better than any description you can write about them.
The Mortal Engines trilogy (Hungry City Chronicles) by Philip Reeve is an under-appreciated series. Good for about the Harry Potter ages. The first line is a classic: “It was a dark, blustery afternoon in spring, and the city of London was chasing a small mining town across the dried-out bed of the old North Sea.” Supposedly, Peter Jackson has the filming rights, though there is no indication he will do anything with it soon.
Another blast from the past just hit my brain - Tom Swift. Many many series of books starting in 1910 starring both Tom and Tom jr.
I read Philip Pullman’s His Dark Material books as an adult; as an English geek I loved the allusions to Paradise Lost.
My son read Patrick Rothfuss’s The Name of the Wind at 11; it was, no kidding, an epiphany, although it’s not really a kid’s book, more a coming-of-age book.
Cherry Ames! Lord, I had my mother’s copies of Cherry’s adventures as Chief Nurse, etc. Produced a short-lived and ill-founded ambition to become a nurse. Nancy Drew, too, the early ones (blue hardcovers, anyone?).
Alas, those old series books (Hardy Boys, Nancy Drew, Bobbsey Twins, Cherry Ames, etc.) don’t hold up too well. I couldn’t get my kids interested in them at all. They are fun to collect, though.