Which words don't get capitalized in the title of a book?

I was looking at this title:
Bottlemania: How Water Went on Sale and Why We Bought It
and it occurred to me that I’m not sure what the official rule for this is – why is “on” lower case but other short words like “we” and “it” are not – is it just the prepositions that are lower case? Or is there a more in-depth rule? I’ve always just tried to do what instinctively feels right – keep the prepositions in lower case – but maybe there’s a different rule at work? and while we’re at it – is lower case two words? or is it lowercase? Where’s Gailcalled when I need her?