Librarians like to cite titles without capitalizing any word but the first and any proper nouns. Thus The History of Tom Jones, a Foundling, is known in library catalogues as The history of Tom Jones, a foundling. This saves wear on the shift key and is not generally objectionable, but it does require some care in distinguishing proper nouns. For example, there is a world of difference between An introduction to Vulgar Latin and  An introduction to vulgar Latin.