In library catalogs, pieces of music are given added, preferred titles, so that one search finds every version of a piece. Preferred titles (aka uniform titles) are especially useful because pieces could have: titles in multiple languages (Brandenburg Concertos vs. Brandenburgische Konzerte); form titles and numbers (Sonata no. 3); well-known but unofficial descriptive titles (Moonlight Sonata); collective titles (Songs; Works).
Need to check a title or opus number? Use reference sources such as:
Formerly Grove Music Online, it offers full text of the 29-volume print edition with the added benefit of sophisticated search capabilities, one-click cross-referencing, and an ever increasing network of web-links to musical sites around the world.