Batman: The Long Halloween is the title of a 13-issue comic book limited series written by Jeph Loeb with art by Tim Sale published by DC Comics in 1996 through 1997, from Halloween to Halloween. It was inspired by the three Batman: Legends of the Dark Knight Halloween Specials by the same creative team, and made possible at the instance, and under the guidance of, then series editor Archie Goodwin. The entire series has been collected in trade paperback edition, this one. Taking place during Batman's early days of crime fighting, The Long Halloween tells the story of a mysterious killer who murders people on holidays, one each month. Working with District Attorney Harvey Dent and Lieutenant James Gordon, Batman races against the calendar as he tries to discover who the killer is before he claims his next victim each month. It also tells the origin of Two-Face, adding along to the story in Batman: Annual #14. In continuity terms, The Long Halloween takes place after the events of Batman: Year One, showing the transition of Batman's rogues' gallery from simple mob goons to full-fledged supervillains. Several stories definitely take place in between Year One and The Long Halloween, including Batman and the Monster Men, Batman and the Mad Monk, and Batman: The Man Who Laughs, the last of which tells of the first encounter between Batman and the Joker, following up on the events of Year One. It has been accepted into continuity after Zero Hour erased the events of Batman: Year Two from the canon. This is a prime example of what a Batman story should be and how it should be done. Harry Goldstein of IGN after a positive critical review would rank The Long Halloween #5 on a list of the 25 best Batman graphic novels (at the top of which is Batman: Year One) of all time.
NOTE: This continues my modest attempt to tell the stories of Batman in the chronological timeline starting from the continuity set by Batman: Year One. The graphic novel has been packed into a two-part RAR archive. You need to have both parts in one folder to extract. Interchangeable links.
If you don't mind running into some spoilers, I would recommend reading the articles found: @http://comics.ign.com/articles/621/621334p1.html @http://comics.ign.com/articles/624/624619p1.html and @http://www.the11thhour.com/archives/062000/comicreviews/batman.html.

A costumed hero learning he can... A serial killer using the holidays to mark his handiwork... A crime lord trying to hold onto a crumbling empire... A city beset by gangsters becoming a haven for freaks... An honest district attorney hiding a terrible secret... A dark woman tempting the Dark Knight detective... And a friendship that would be shattered forever.
These are the pieces of the murder mystery: The Long Halloween.
Created by the highly acclaimed and Eisner Award-winning team of Jeph Loeb and Tim Sale (authors of Superman For All Seasons), this thrilling story features some of the most deadly and cunning members of Batman's rogues gallery and a dramatic retelling of the origin of the tragic villain Two-Face. -- blurb found at back cover