If you loved the characters just from the movies, then you'll adore the books. The movies are a bit crap in that regard as from the books, you get so much more of their personalities. I would that the whle point of the books is actually the characters, not the magic or even the background main story (harry having to fight voldemort till death) which is really a background story - as in you hear about it very rarely - They're actually really funny books, especially the second one (in my opinion)
Digi,
Basically the Order of the Phoenix is... uhm let me think i can't really remember....
SPOILERS IF YOU HAVEN'T READ OR WATCHED THE GOBLET OF FIRE.
Right. After the end of the Goblet of Fire, it becomes public that Harry "allegedly" fought with Voldemort. I say allegedly because the whole theme of the book is that no one actually believes him except his usual supporters. It's a bit crap, it felt very different because everyone is against Harry in this book, and he's all angry and moody and annoying, and it just didn't feel right (as i said, it's illegal to dislike Harry but i think they let J K Rowling off for this one). Everyone is in denial apart from Harry's supporters (dumbledore and a few others). Heck, some people even report on the national magic papers that it was Harry who killed Cedric at the end of the Goblet of Fire!
Anyway, everyone is on Harry's back, there are new horrible (but VERY funny) teachers and the whole book is spent abusing Harry (pretty much...) and his supporters. Even good old Dumbledore isn't nice to him! Till the end of course, and it all gets explained of course...
The book is VERY long and considering nothing much happens (compared to the past books at least), it can be quite boring at times... It's still good, but definitely the one i liked the least. Not sure the movie will do too well because of that in fact, unless they REALLY go all out on the characterization which should be awesome and is pretty much the only good thing about the book. There are some seriously awesome characters, especially Luna (think a Buffy's Anya but in the Harry Potter universe: priceless!) who has some seriously hilarious lines.
Oh and if you're asking, the Order of the Phoenix is a secret society which Dumbledore founded (The Phoneix is Dumbledore's symbol, remember it from the movies? He has one in his office and it saved the day in the Chamber of Secrets) when the war with Voldemort started years ago (before all the events in the Harry Potter books, the war which ended when Voldemort tried to kill baby Harry but couldn't, disappearing for a long time...). It's the Good Guys(TM) and the direct enemies of Voldemort's group of killing mates (they have a name... the micky mouse club? no... Take That? no... can't rememeber but it has "death" in the name). Their HQ is the only place Voldemort can't get to because it's magically hidden (think a dimensional kind of hiding place).
Oh and now people can teleport. I mean, they always could, but it becomes more part of the story from this book until then next when the boys actually learn how to do it (or is it in this book? can't remember), and then Book7 when they will get their licenses apparently. Yes, you need a license in the books to teleport around the world... Really...
I'm sure someone else will give a better summary...