The Long Halloween is a follow-up to Year One, written by Jeph Loeb because Frank Miller had no apparent interest in doing so. (After having read All-Star Batman and Robin, I think that's a good thing.) It essentially picks up right where Year One left off. Batman is still kind of new to all of this, still working out his code of ethics and how best to use the Batman persona. It's also a passing of the torch for Gotham itself, moving the city from one ruled by the mafia to one terrorized on a pretty frequent basis by nutjobs in masks. I have to wonder what the average Gothamite would think about all this, and if they'd rather have Falcone or the Joker. It's also a detailed origin story of Two-Face. It sounds like a lot going on, but there's plenty of space in 13 issues to cover it all.
Everything in the first paragraph is actually secondary to the real story, a mystery revolving around a serial killer dubbed Holiday because s/he kills only on holidays, leaving behind a token related to the day. It's a really, really good mystery. It's entirely engrossing, and the resolution is surprising and satisfying.
My only issue with the book is that Tim Sale's art just doesn't work for me. Never has, and I doubt it ever will.