This topic is a bit lost here in Critique; it might have gotten more response if placed in General.

So far as I know, the use of "title case" for book titles has been normal in the English language since the eighteenth century. If I had to guess, I'd speculate that it derives from the earlier practice of using initial caps on words for Emphasis, and that by the over the course of the eighteenth century it was started to become normal to retain this custom for book titles and certain other kinds of uses (names, offices, etc.) but to use lowercase in most other circumstances (although that evolution continued during the nineteenth century).

There are some specialized situations where it is no longer the custom in English, especially library catalogues and certain bibliographic styles.