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

Young adult means just that- adult themes toned down (and sometimes not all that much!). Personally I'd say 9 is a bit young- I'm just starting to recommend young adult to some 14yr olds who I think can cope with issues. As PPs have suggested what about classic 'children's lit'. Some of the language is so far removed from ours that she'll be working quite hard to keep up with plot, which itself will be very accessible.

Oh and City of Ember

I have two DSs who never stop reading.

DS2 is 9 Ds1 is almost 12.

I would recommend

The Red Pyramid Series (by author of Percy jackson)
The Secrets of the Immortal Nichols Flammel series
Small change for Stuart/Big Change for Stuart
Varjak Paw
Time travelling with a Hamster
My Brother is a Superhero (and follow on book)
Possibly - The Boy in the Tower if they are ready for an introduction to end of the world/dystopian stuff
The Divide series
The Never Ending Story
Momo
A Letter for the King/ The Secrets of the Wild Wood
Hugo Cabret/Wonderstruck/The Marvels - all for their sheer beauty if nothing else
George's secret key to the universe series (Stephen Hawking)

I'm an editor and I work a lot on YA fiction. Echoing others here: the themes in YA fiction are often totally unsuitable for 9yos, even ones who are good readers.

It's not about being able to decipher and decode the words on the page; it's about being able to handle the themes, feelings and topics dealt with. YA books can be about sex, drugs, refugees, abuse, death, illness, war, fighting, nuclear war... Many are post-apocalyptic or dystopian (and hard work for adults to read - Hunger Games and its ilk).

I'd recommend perhaps trying some older books which may be written about innoccuous subjects but in a harder-to-access style, thus widening your ds's reading experience.

My ds (9) enjoys Michael Morpurgo, Helen Moss books (Adventure Island and the Secrets of the Tombs series), Crowns and Codebreakers series, The Accidental Prime Minister series, and Lauren St John books (The Last Leopard, etc.)

I'd go with bluechampagne's list and add the belgariad series (david eddings) and maybe the pern books by Anne mccaffrey.

DS1 is 9 and an advanced reader. Here's what has been enjoyed recently:

The Dark is Rising
A Christmas Carol
The Wolves of Willoughby Chase
Heidi
The Wee Free Men
The Box of Delights

Cathy Cassidy for edging towards YA, and any Michael Morpurgo. Still loves Wimpy Kid, Dennis the Menace and How to Train your Dragon for light reading in between the heavier stuff.

He's read all the Harry Potters and all the Roman Mysteries.

Agree with mummytime that older books might be better for now than those labelled YA. I loved "Mortal Engines" but wouldn't suggest it to DS1 yet. So ...

The Secret Garden
Swallows and Amazons
Children of the New Forest
The Children of Green Knowe
Elidor
A Little White Horse

Dd2 got into Josephine Tay at about that age.
I'm not sure exactly where they'd be put in category wise.
I was looking for the copy of Daughter of Time (I think that's it) for ds (now 9yo) as I think he'd love it too, but she tells me she read it to death.
She also enjoyed Brat Farrar and, I think the other one she really enjoyed was The Franchise Affair.

Ds reads adult fact books, and has for some time. Mostly about WWII and/or planes but has recently become interested in spying (due to Alex Rider). His factual reading age is far above his fiction one, and always has been since he was 3yo and learnt to write Concorde, Red Arrows and Sonic Boom before any other words.
I just have to try not to roll my eyes when he comes at me saying "You know about the " and describes it step by step.

I agree with the comments of Young Adult books. My girls looked at them and decided to bypass them as they tend to be overly about romance or gritty living, and neither of them is particularly interested in that.
Looking back I don't think I particularly used that section of the library either.

I would be careful - dc1 (13) is an extremely clever, high achiever and still loves to curl up with good old diary of a wimpy kid sometimes.
I would recommend Michael morpurgo though.

I would avoid anything called "Young Adult" as that tends to have too much sex and drugs and violence or "grown up themes" like dying of Cancer. So beware.

Admittedly I got a shock when I was reading Alex Rider to my son and one started off with him chasing after drug dealers - and having to give a quick drugs talk on the hop at 8 pm with no warning...

Classics, old fashioned childrens books and the "harder" 8-12 books are your best bet (including Alex Rider just beware).

My ds is also nearly ten and is a avid and advanced reader.
He has recently read LOTR (after whizzing through the Hobbit) and also read all of the lemony snicket series over Christmas.
Others he had loved in the past include Enid blytons afventure series, animals of farthing wood, the land of stories series, Alex Ryder, The boy in the tower (fab, I read it after him), the previously mentioned Pratchett ones, Cheese, Wildwood.
He didn't like Percy Jackson.

He still re-reads his beloved Beast Quest books the whole time too!

W H Smith

What smith categorises it's books well if you go into one of their shops. They keep the young adult books well away from children's and teenagers books because they are for older teens and, well, young adults. Libraries aren't all as good at categorising the books well and lump too much into teens with no distinction between what's suitable for an 11 year old Vs a 16 year old. I tend to use online lists to gauge appropriate age and order books from the library. Lovereading4kuds is a site I use.

I think it's a tricky age I had to work hard to find books for DD'S between ages 10 and 12.

Mortal Engines has got some quite emotionally tough stuff in the later books though. Not the "I'm doing crack, cutting myself and having sex with my teacher because nobody understands me" stuff that you sometimes find in "YA" books if you're not careful about vetting, but still beyond the range I'd consider suitable for a 9 year old.

A word reading age 4 years above chronological age happens quite often ( I'm an ed psych, I see this a lot; I think that once you get to a critical mass you can just read most words. I wish schools would stop using age equivalents though as they can be very misleading for several reasons.). His 'score' doesn't tell you everything about his reading comprehension skills, especially the higher order ones and you suggest that in terms of themes, your son is well suited to books suited to children of his age. In which case, think about broadening the range of books he reads, rather than going for YA books. My daughter is a good reader at 7 and enjoys poetry ( Old Possums etc) and likes older books aimed at children such as the Just So stories. At the moment she's reading JRR Tolkein's letters to Father Christmas and she's eyeing up Michael Morpurgo's rewriting of Grimm's fairy tales next. The content is appropriate but they provide a different experience. She's only just started Harry Potter ( although now is on a book a week) as she tried them last year and was a bit upset by the themes.

However, I would also add that I am extremely grateful that as a good and early reader, my parents just gave me free run of their bookshelf and the local library. By eleven I was quite partial to Ian Fleming, Agatha Cristie and a bit of sneaky Jilly Cooper (among more highbrow works now and again). YA didn't exist as a genre then, and in a way I'm quite grateful for that.

DD 10 is enjoying The Last Dragon Chronicles by Chris D'Lacey.

DS (similar age) has read most of the Percy Jackson books and is now reading some books about children at Bletchley - Secret Breakers - which he's enjoying. Gone through a lot of Philip Pullman and Terry Pratchett which have also gone down well.

I want him to read Philip Reeve's Mortal Engines series next.

Loads of good suggestions here - would also suggest looking out for non-fiction in areas that interest him as it's likely to be 'safe' while more challenging to read IYKWIM. So eg if he has a sport he enjoys, maybe some lighter popular science if he's that way inclined.

I'd tend to avoid things badged 'YA' as they're often relatively simple in structure/language but with content aimed at teenagers.

My just turned 10 year old has read some of the younger age pratchett books including the short stories such as dragons at crumbling castle, the hobbit, goodnight Mr Tom, pull man's adventures of the new cut gang. Things like that. He likes war stories and has read a few morpurgo books too. Plus Harry Potter etc.

Diana Wynne Jones - the Chrestomanci series is probably a good place to start but she wrote about a million books and they're all great.

Yes Percy Jackson has gone down well with my nearly 9 DD. She is just about to read her first Terry Pratchett - hopefully it will be a hit as he's written quite a few for that age group.