Is it true that Rowling is going to write an Encyclopedia of Harry Potter world?

Yes it is true - but she hasn't said how long it will be before she releases it, because she wants it to be thorough. But she has said she will not hide anything, all the details in her notes will go into it - I am so so so excited, I can't wait!! After DHP2 is released, the world will be thrown into darkness, but at least there will be something to look forward to!