You should go over to comicvine. Just saying. They got a whole wiki about this stuff.
DC has repackaged Batman: Hush a bunch of different ways. If you just want the story sans extras, the regular Batman: Hush paperback is fine.
Heart of Hush isn't really a formal sequel to Batman: Hush. It's just the next major appearance of the character. I remember it being quite good.
Hush is hyped so much because Jim Lee drew it, I personally hate his art almost as much as Liefeld; but I recognize that I am in a very small minority in that opinion.
Check out the Long Halloween / Year One / Killing Joke / Dark Knight Returns if you want to read good Batman stories.