When Pottermore got started, this blog was a way for people who had not yet gotten their letters to take advantage of those who had. Now that Pottermore is open to the public, it's more of a glossary, a compendium of all the exclusive new content in Pottermore, all in one easy place.
You'd think I wouldn't need to say it, but if want to experience Pottermore for yourself without it being spoiled, DO NOT READ ON. Obviously.
Chapters 5-11 of the Chamber of Secrets are now available to be explored on Pottermore.
As of Wednesday, July 11th the second book, Harry Potter and the Chamber of Secrets, is open on Pottermore.
EDIT: Only the first four chapters are open.