Just to support other editors, at Grade 9 I teach and have taught Cry, Beloved Country, Romeo and Juliet, Lord of the Flies, Animal Farm, Oedipus Rex, A Separate Peace, Diary of a Young Girl, The Outsiders, plus a range of short stories and poems. It is important though that these works are accessible and not too subtle - students must be able to apply abstract terms such as allegory etc to them.
I teach Pre-AP Freshman English and this year we are reading To Kill a Mockingbird, Romeo and Juliet (of course!), Oedipus Rex and a Farewell to Manzanar. There are AMAZING resources on this website to help you with most of the novels that people have listed in response to your post.
Just keep in mind that you don't have to pick "easy" stuff for them to read, you will just have to scaffold more and provide more enrichment activities.
I love Great Expectations, The Outsiders, Flowers for Algernon, Diary of a Young Girl, and Romeo and Juliet. Also, I try to fit in a comedy from Shakespeare: Midsummer Night's Dream is always a good one, as is The Tempest and The Taming of the Shrew.
It's been awhile since I taught 9th grade, but we used to do The Contender, and the kids seemed to like it. I also have a few copies as free reading books in my classroom, and a couple of my 7th grade boys picked them up last year and read them of their own free will--which was pretty exciting to their teacher! I've never seen a group of kids who didn't like The Outsiders, and although it's traditionally taught in 7th or 8th graders, I've also never had a class who didn't enjoy Roll of Thunder, Hear My Cry and the sequels.
I enjoyed pairing novels with plays, poems, other novels, and short stories (as opposed to just reading a novel straight through in relative isolation). Zlata’s Diary is a novel that I have used successfully with ninth graders and paired with The Lottery Rose. For both novels, I used the theme of man’s inhumanity to man as the guiding theme. The cruelty of war compared and contrasted with the cruelty of child abuse makes a powerful thematic study for the two texts.
I have an additional question. You said you are ordering novels for your ninth graders but did not specify if you are ordering class sets that are required reading as a group or if you are ordering books for them to read on their own. Either way I am sure you want to have good literature that will foster discussion and good examples of writing and all of it's elements. It's also so important, though, to find literature that your students will read. You can introduce the best authors, the most renowned, the most praised, but if kids won't read it, there is nothing to teach. I have taught 8th grade advanced English for six years and have spoken with students that return once they go on to high school and a few novels that I have seen success with are An Acquaintance with Darkness, The Hound of the Baskervilles, The Chocolate War, Anthem, Great Expectations, To Kill a Mockingbird, The House on Mango Street and Their Eyes Were Watching God. I think it is extremely important to find books that can be cross curricular and novels that are diverse to interest the many students that will come to your class.
Based on my own experience, I have to agree with timbrady. Finding quality, topical novels for freshmen that the majority, at least, will find interesting is the best approach, I think. Some titles that come to mind are Flowers for Algernon, The Outsiders, Bless the Beasts and Children, The Light in the Forest, A Day No Pigs Would Die, Night, Jonathan Livingston Seagull, Holes, and Johnny Tremain. My students especially enjoyed Jonathan Livingston Seagull, Flowers for Algernon, The Outsiders, and The Light in the Forest. They're easy reads that promote a lot of discussion and good writing topics.
As the previous posts suggest, there should be some networking between you and the other teachers (those in younger grades and those in older grades), so that your ordering of novels can fit in a successful sequence that students will experience.
Having said that, I think that "Inherit the Wind" or "West Side Story" can serve as excellent examples of reading plays in class. They are also fairly active, lending itself to 9th grader appreciation. A challenging book that can really be rewarding would be Paton's "Cry, the Beloved Country." The standard in 9th grade in our district used to be "A Separate Peace." I think examining what students have read and seeing what your students will be reading can assist you greatly in determining what they should read this year.
I would suggest that you find out what they are reading already (from those that are reading), and use those books. I think that 9th graders should be introduced to the enjoyment part of reading at that age ... leave the other "stuff" for their junior/senior years. This is more difficult to do than it sounds. I did this one year, and found out that if you can get 50% of the students to like any one book at one time, you're doing well. The students that liked sports hated fantasy; the students that loved fantasy ... well, you can figure it out.
I know that a number of people will disagree with this, but I think extensive and enjoyable reading is the most important thing we can do for our younger students.
This doesn't mean that you can't try a classic; I read Great Expectations as a freshman in HS and loved it. But, of course, I wound up being an English teacher :)
Your order really depends upon which type of ninth graders you will be teaching. If they are honors freshman, Animal Farm is a definite for our school. It offers students a solid background of allegory and political satire.
If you are ordering for struggling students, we have had much success with literature circles connected to major works such as Romeo and Juliet. For example, we have students read Son of the Mob and other modern versions of the play when we study Shakespeare.
A great book that comes off of the Florida Teen Reads list of 2012 would be Ship breaker or Escape from Camp 14, which is from this years teen read list. Out of these to I highly recommend Escape from Camp 14 since this details the life of a defector who was a prisoner in one of the concentration camps in in North Korea. It really is a great eye opener for 9th graders especially since things like the Korean War are rarely if ever mentioned in textbooks and many people tend to handle discussions about North Korea with poor information.
In my ninth grade I read Romeo and Juliet, Fahrenheit 451, and Animal Farm and these are all books that is a great start to literature. All these books provided handfuls of themes and a lot we could learn from this. We had projects that related to each book and beside books read, we analyzed the movie differences as well as watching Star Trek.