Digitized image from 1805 Mozart Thematic Catalogue, Verzeichnüss aller meiner Werke. https://blogs.harvard.edu/loebmusic/2013/08/15/newly-digitized-1805-mozart-thematic-catalogue/
If you are studying the works of a specific composer, chances are you might have reason to use a thematic catalog. These are reference works that list the complete works of an individual composer, often providing important information including but not limited to:
While composer thematic catalogs are perhaps the most common, there are also thematic catalogs featuring works by multiple composers based on a single genre or type of work, such as this Dictionary of Opera and Song Themes, Including Cantatas, Oratorios, Lieder, and Art Songs.
When looking for thematic catalogs, be aware that the title will not always include the words "thematic catalog!" Look for books with call numbers beginning with ML134, or titles using phrases like "catalog of works," "catalogue raisonné," "Thematisches Verzeichnis," or "descriptive catalog." You can also use the genre filter for "Thematic catalogs (music)," which will catch many but not all thematic catalogs.
While thematic catalogs are very helpful for those studying canonic composers and topics, unfortunately they don't always exist for all composers. If you are passionate about promoting the works of underrepresented composers, please consider working on a project to create a thematic catalog for a composer who doesn't yet have one.
See below for some examples of interesting and/or unusual thematic catalogs.