The Long Halloween is a follow-up to Year One in that it's the next major storyline and it uses many of the same characters and themes from Year One. In the Batman chronology, though, it doesn't happen until one or two years after Year One. In between is where all Batman's supercriminal rogues start appearing. Batman's first encounter with the Joker takes place in the graphic novel Batman: The Man Who Laughs, as well as its immediate sequel, Batman: Dead to Rights. His first encounters with the Riddler, Poison Ivy, and the Scarecrow are shown in the collection Batman: Four of a Kind (although if I remember correctly some of those stories are ambiguously canon). There's also a Scarecrow: Year One comic, but I'm pretty sure it's out of continuity because of a contradiction with Two-Face's timeline or something. Mr. Freeze is not in The Long Halloween but he is in its sequel, Dark Victory, which you will probably want to read next, and he first appears in Batman: Snow (although I'm not sure if that story is completely canon either). But you don't really need to read those other stories to understand The Long Halloween, just know that all these crazies have started showing up since the end of Year One.