I'm looking for a list of books appropriate for a precocious nine year
old with a reading level of a seventh grader. The important thing is
that the books not veer into topics that require the emotional
maturity of a twelve year old.

I would recommend Christopher Paolini's Inheritance series. I don't
think it requires much emotional maturity, but it has been awhile
since I read them.

tisme-ga

http://semicolon.reachcoop.org/?p=1313
Boys? Week: Nine Series for Nine Year Old Boys

One of my nephews highly recommends Al Capone Does My Shirts by Gennifer Choldenko.

http://www.crescentblues.com/7_11issue/bk_Chold_capone.shtml

Artemis Fowl  by Eoin Colfer   a set of five books so far

... a precocious 9 year old boy or girl?
Some girls' books from decades ago require a higher level of reading
whilst catering for a less sophisticated level of maturity. (e.g. Anne
of Green Gables series; Little Women series; Secret Garden; etc.)
Loved 'em. Ah, the good old days.

Books by Ernest Thomas Seton, if he is interested in animals.  ETS's
"Rolf in the Woods" I liked at that age, American boy with an Indian
during US Revolution or French Indian War, but that is the period, not
the subject (don't know if ETS's books are still in print).  Yes:
http://www.amazon.com/gp/product/1885529090/104-8886583-3508735?v=glance&n=283155

Holling C. Holling has a great series of books that are informative
about areas of the US with great illustrations:  Seabird, Paddle to
the Sea, Tree in the Trail, Minn of the Mississippi, are the titles I
remember.
http://www.recess.ufl.edu/transcripts/2005/0802.shtml

http://www.bywayofthefamily.com/seriesresults2.cfm?strSeriesCode=Holling

All are recommended on a homeschool website. I forgot that I also read Pagoo.
The emotional content of all the above is no problem.

Just buy the books and look at them yourself and leave them around. 
If your son is not interested at the moment, I expect that he will
find them when he is.
And encourage him to got the the library and find whatever interests
him.  It doesn't have to be fiction.

Mark Twain:  anything!  Tom Sawyer, Life on the Mississippi, ...

O. Henry's short stories are also delightful.  I think I read them
all.  If there was any possibly troublesome emotional content, it went
right over my young head.
http://www.kirjasto.sci.fi/ohenry.htm

Give him his own subscription to a magazine that you find appropriate.
I devoured Popular Science for years.  Maybe it has changed, but you
can find out here:
http://www.popsci.com/popsci/currentissue/

Boy's Life, the official mag of the Boy Scouts of America, may be if interest:
http://www.boyslife.org/

Receiving his own magazine in the mail will be a great incentive to read it.

And if it is part of your life, give him a Bible and a child's guide
to it.  Here is one guide that appears on several websites:
http://www.eden.co.uk/shop/childrens_guide_to_the_bible_44148.html

Again, show your own interest in the books and let him discover them.

I can't stop.  :-)

Kipling: anything!  Boy Stories, Kim, the poems.

Poems:  R. L. Stevenson's "A Child's Garden of Poems", maybe to be
appreciated together.

And one that that was given to me when I was ten (just checked, Thanks Mom):
"Big Tiger and Christian", by Fritz Mühlenweg:
http://www.amazon.com/gp/product/B0006AT4IS/104-8886583-3508735?v=glance&n=283155

I can only second Liam Keeley's review.  The book is partly
responsible for my interest in the Orient and the greater world in
general.

I've said it already, but the best way to get a child interested in
books is to have them available and let him see your own interest in
them.

This has been fun; I hope it helps.  :-)

Goosebumps
Deltora Quest
A series of Unfortunate Events
Unreal etc by Paul Jennings

This is what my 9 year old boy reads

Cheers

A few titles/series I read to my kids or that they devoured themselves:

The Hobbit, Tolkien
A Wrinkle in Time/A Wind in the Door/A Swiftly Tilting Planet, Madeleine L'Engle
The Dark is Rising series (5 titles), Susan Cooper
Redwall series (many titles!), Brian Jaques
The Railway Children, E. Nesbit
Swallows and Amazons (beginning of a series), Arthur Ransom
The Sign of the Beaver & other historical fiction, Elizabeth Speare
Island of the Blue Dolphins, Scott O'Dell
The Perilous Gard, Elizabeth Pope
The Princess and Curdie/The Princess and the Goblin, George MacDonald

I also found this online list of books for gifted 9-year-old readers;
there are many other well-loved books/authors on this list that my
children and I enjoyed.

http://www.nswagtc.org.au/info/books/Books9.html

You can find a number of other such lists by doing a simple Google
search for books for gifted readers. The one at the link above seemed
quite comprehensive and well-rounded.