There is no single rule for English title case. Most build on title-casing of individual words, so the rules for that first.
- Generally, one capitalises the first letter, and lower-cases the rest.
- Traditional lower-case abbreviations are sometimes not capitalised at all (e.g. op. cit. etc.) and sometimes initial-capitalised, but generally not used in titles anyway.
- All-cap abbreviations remain in all-caps.
- Words which have a partial capitalisation including both initial letter and another when normally written retain it, e.g. McLeod, O'Reilly, and some trade names.
- Words which have a partial capitalisation not including the initial letter may be written with or without capitalising that initial letter, that is some styles would write d'Este and some D'Este. Typically we do the latter in styles that would capitalise prepositions as described below.
There are other rules in other languages, such as Dutch words beginning with ij capitalising both (e.g. ijsberg becoming IJsberg) and some languages mutate words by prepending letters which are never capitalised. Such foreign rules may or may not be followed with foreign words borrowed into English, though it's generally considered correct to do so, and can even affect meaning if you don't (nAthair in Irish means father while Nathair means snake).
With those rules for title-casing words in place, some common rules for whole titles are:
- Upper-case the whole thing.
- Title-case everything except articles, prepositions, and conjunctions unless they are adjectival or adverb use of prepositions.
- As above, but also capitalise prepositions and conjunctions longer than four letters long.
- As per 2, but lower-case all prepositions.
- As above, but also lower-case other determiners like numbers.
- Lower-case the whole thing.
- Capitalise all nouns (more common historically than now).
- Title-case every word.
And there are yet others found. In addition, one might decide that a word that isn't title-cased is important to the title for some reason, and then title-case it. One might also capitalise after an initial Yes or No or if the title is an apostrophe starting with the word O, one might capitalise after that (so one could have "O For the Wings of a Dove").
One strong advantage of number 8 is that while it is often considered rather uncouth, it is easy to do programmatically. It's often found when some computer code is "fixing" titles from a source of mixed quality. Such computer code need only look for word boundaries, upper case the first letter and leave the rest as is. It gets a bit more complicated if your sources might have some all-capital phrases, but is still much simpler than having computer code determine if a word is a preposition, never mind whether it's being used in an adjectival or adverbal use or not.
So we can expect to see quite a few different variations coming from both human and electronic editors.