In English literature, improper capitalization of some phrases is common. A friend of mine calls them 'Milnesian Capitals'.
These tend to be used in late 19th early 20th century British writings. A.A. Milne wrote that
'[Winnie-the-Pooh] was a bear of Very Little Brain.'
Terry Pratchett uses Milnesian Capitals frequently, particularly in dialogue.
Are there any rules or conventions governing their use? Is it an older style that has been discarded?
Best Answer
In the example you give, it is used as a type of irony. Normally the title case is reserved for titles, but in that case it is used to emphasize the fact that "Very Little Brain" is Winnie-the-Pooh's identity.
It can also be used in a more serious manner to give importance or reverence to certain words. For example, in the Christian bibles, all pronouns referring to God are capitalized (He, His, etc.). Particularly in older texts, you will see this capitalization extended to other subjects that are meant to be of great importance or revered.