All of my choices are not books about bullying as such but still ones in which it plays an important part. I had wanted to include The Killer's Daughter by Vivian Oldaker but The Mole picked that first!
Be warned there are probably plot spoilers in this post - though not in the individual reviews.
A Million Angels by Kate Maryon - Mima is depressed and lonely since her father has been posted to Afghanistan. First her best friend Jess starts to torment her with ghastly accounts of what may be happening to her dad, then school bully Tory begins to pick on her as well. The mild teasing turns nasty when their parents mistakenly encourage the girls to spend more time together. The story shows how parents too easily pick up the wrong idea and make matters worse, even though they feel they are helping.
Boys Don't Cry by Malorie Blackman behind the main plot of teenage parenthood is a tale of gay teenager Adam coming to terms with being 'different' but bullied for it. He attempts to hide this from his, mainly accepting, family but the reader is aware that things are growing unpleasant for him. The ugliest aspect is that someone is joining in with the bullying in an attempt to hide their own feelings and sexuality.
Torn by Cat Clarke - how NOT to tackle bullying! Tara is the popular, bitchy queen of school, out to belittle and humiliate anyone not in her clique. She wasn't always like this though, so how did she change from nice girl to bully? Also, if you feel you're being bullied, don't go down the route picked by these girls!