In the end, it is the publisher's decision, but the usual practice is to capitalize the first word and all but the least important words (articles and prepositions mostly).
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Hello,
English does have many capitalization rules. In what book titles are concerned, the first word and all content words of the title are capitalized. That is why in your example the preposition "of" isn`t capitalized - because it isn`t a content word as it does not have any independent lexical meaning.
Best wishes,
Diana