Halloween is a huge thing in L.A.,” says Christina Rice, a senior librarian for the Los Angeles Public Library—and she has proof. She oversees the library’s photo collection—an estimated three to four million snapshots of life in the city, housed on a single floor at the Central Library Downtown. The collection was founded in the 1940s to document the rapidly changing city, and has grown through donations ever since. It includes both major news events and less-major ones, including trick or treating traditions of various neighborhoods. “The scope of the…