Can you recommend Young Adult books suitable for a 9 year old?

Oh yes, DS1 LOVES Lewis Carroll's Alice. He also tore through the Unfortunate Events series.

How about:
Sherlock Holmes
The Prisoner of Zenda
The Scarlet Pimpernel

Gallivanting. Fair point. Maybe we should start a new thread on picture books suitable for Trump? I'd start with The Piggy Book by Anthony Browne. (This by the way is the best book ever and a must for a nine year old boy too!)

I didn't know that OP but I'm really pleased. Mine have loved all of his children's books. The Christmas books are amazing. We read them aloud as a family last Christmas and this Christmas and we all really enjoyed them. Ds2 also had the runaway Troll and Shadow Forest for Christmas.

Tea- but Trump doesn't read. He gets all his info from watching tv. There are rumours he's functionally illiterate.

I loved A Little Princess too. My favourite childhood book.

There are some great new books out on kindle that have a grown up feel but are still age appropriate. It's a trilogy called Cat's cradle trilogy and without wanting to sound sexist the first two are probably aimed more at pre teen girls but the third one is more of an adventure and deals with intelligent issues like cloning. The main characters are travelling around the world so if he is interested in different countries he might enjoy it.
The main message is that we are all the same underneath. Shame Trump hasn't read it. It's Cat Lives by T. J. Healy.

Lloyd Alexander, book of three etc. Vaguely Lotr ish but based on Welsh myths and legends, with the main character growing up 5 books. Watership down. Loved, loved, loved that at 8 and still read it on occasion. The Indian in the cupboard series? Or a bit young?

I'm not sure if anyone has mentioned Noel Streatfield? Books like Thursdays Child / The Growing Summer / Apple Bough etc might appeal.

I saw Eragon recommended upthread - dd was given the series as a 9th birthday present and I was quite but she really loved them and they got read and re-read.

I do agree with Sadik.
My dc have generally enjoyed much more the old books.

Biggles, Chalet School, Lone Piners, Blue Door Theatre, Punchbowl Farm, Stormy Petral etc
Those are the books they read and reread. Modern books get read once and offered into the Charity shop bag generally. Alex Rider an exception, and dd1 loved Cherub etc.

Ds1 loved the tripods trilogy but DS2 was freaked out by Chocky. I really do think you have to be careful with this age group. I have friends who have chucked things like the Hunger Games, Maze Runner and the Gone series at their 9/10 year olds without even thinking to read them first. They are all completely unsuitable in places. People seem to think that because the books are about teenagers then they are ok for 8-10 year old children. They are not.

Mine love skulduggery pleasant and artemis fowl is fantastic, we all enjoyed the whole series on audio book as a family. Matt Haig is great when writing for children too (and when writing for adults for that matter - read The Humans - fabulous book)

His Dark materials is one of my all time favourite series but the final book would be tough going for a 9 year old. And if you are very religious you might find it offensive. We have again done the series by unabridged audio book as a family (we spend a lot of time in the car). Northern Lights is a work of genius. The Subtle knife is good but not as good and The Amber Spyglass was put on hold for a couple of years until Ds2 is a bit older and can both appreciate the story and understand it.

There are so many amazing childrens series out there. I just don't understand why people give their children YA books. And I'm in a similar position to you OP where mine both have reading ages 4-5 years above their age (which in any event remember is based on averages and so doesn't necessarily mean what it appears to mean at first since some children are very poor readers)

I think some of the more dated stuff actually puts children off. A Little Princess was my absolute favourite book as a child and still works. On the other hand, I also adored Enid Blyton but much of her work hasn't stood the time and is actually quite poorly written. In any event, something like the Secret Seven would seem to be way to simple for the OP's child. Its more for children of about Year 2/3 age.

I think you have loads of great suggestions though to explore Please do read any young adult stuff through first. And a word of warning don't go on the film versions either. Hunger games for example is far worse in the books than in the films (genetic mutation of murdered children to create half human half animal hybrids which then try to rip other children to bits). Maze Runner has children dying from their flesh melting off their bodies, suicide, murder, drug taking, radiation poisoning - all IMO too heavy for a 9YO.

Has anyone mentioned Molesworth? Challenging in terms of reading ability for deciphering the spelling and arcane vocabulary, funny, good for dipping in and out. Treasure Island, or would that be too frightening? Such a great story.

Maybe it's worth just trying them out and seeing what appeals though Bertrand. DD liked Jennings a lot, her fantasy-hating-super-fussy friend loved Biggles (and his little brother who can't have been more than 8 fought him for them - and given that you can buy them as a cheap set on Book People I'm guessing they aren't the only ones), all the reading children I know have enjoyed the Swallows and Amazons books, just for example.

Also, remember they were dated even when we were children! Something like A Little Princess for example would have been 75-80 years old when I read it as a child, so another 30 years isn't such a big deal necessarily. (Though it reduced dd to a sobbing heap - so perhaps only to be recommended to hard-hearted children.)

It's important to remember- if only to protect ourselves from disappointment- that many of the books we loved feel incredibly slow and dated to modern children. And have very old fashioned vocabularies. My reading children rejected Biggles, Jennings, William and The Hobbit, to name a few- except as read alouds.

Also, I think we often unconsciously lop a bit off the ages we read things in our memories. I vividly remember reading a particular edition of "Alice" when I was 9. But I recently actually found the book- and I got it for my 11th birthday........

Also Diana Wynne Jones - Charmed Life is a pre-runner to Harry Potter.

Derek Landy's Skulduggery Pleasant series is well-written and aimed at 9+.

Chris Bradford's Young Samurai series is also good.

Wonder by RJ Palacio.

What about Philip Pullman, His Dark Materials trilogy?
Eagle of the Ninth chronicles, Rosemary Sutcliffe
Earthsea trilogy, Ursula le Guin.

Sorry if I have repeated previous suggestions.

In some ways I think from a theme perspective you'd do better to move onto adult books. At his age I started to read Asimov's robot books for example. I finished Lord of the Rings for the first time at about that age too. Also Agatha Christie or the other golden age detective fiction Authors. Pratchett is always good (The amazing maurice and his educated rodents to start), if the jokes aren't meant for kids the just don't get them. Douglas Adams too for the same reasons. You could also look at some of Anne McCaffery's Pern books (the Harper hall books would probably be the best starting point) and David Edding's Belgariad series.

From the traditional YA fare I'd look at John Christoper's The Tripods or Prince in Waiting books. But I think you may want to read them first. Also books like Chocky or the Chrysalids by John Wyndham. Jean Ure's books might be good too. Oh and Rosemary Sutcliffe.

From the modern YA stuff: Does Garth Nix count as YA? Mister Monday etc. or the Old Kingdom books are probably good.

In our house the rule is more or less that my DD1 (8) can read any book she can reach. But the YA books are all on the top shelves...

Willard price is very very dated...

Not sure if it has been mentioned but the Willard Price 'adventure' books are really good - two boys who go around the world collecting rare animals and having many adventures. My brother and I read and reread the series at a similar age.

What about books such as The Secret Seven, Famous Five and The Hardy Boys, my boys loved them. Also Roald Dahl. David Walliams. Michael Morpago.

I was an avid reader at that age still am, my poor mother had to keep up with my reading addiciton and keep me away from "questionable" content. That said I was reading fully adult books by 14.

Garth Nix is good for content and difficulty, he has several different series so would provide decent material for a while.
The Hunger Games?
Older books like The Hobbit, Black Beauty, Pollyanna, Anne of Greengables. They will give the challenge but don't have more mature content.
InkHeart series
DoomSpell series
Eragon series
The Book Thief
I read Hitchhiker's Guide to the Galaxy at that age.

What do they normally like to read?

None on my list are what I would call Young Adult. He's too young for Young Adult IMO. Plus there is no need for it since there are loads and loads of books aimed at this in between age.

If you decide to do the Boy in the Tower then I'd read it first. Its a good book. Ds1 loved it at age 11. DS2 would be worried by it at age 9. Its basically the Day of the Triffids for children.