It is not simply whether we are speaking of being enumerable vs. the contained. The crucial distinction, as my friend Amber points out, is in the type of quantity of the contained. Are we speaking of things discreet or continuous in quantity? Of distinct quantities (numbers) in something else, we say "contents." For example, 'The contents of my purse are a pen, wallet, etc.' Of continuous quantities (geometric entities), we speak of "content," as in 'The content of my glass is milk' (in a cylindrical shape).

With books, we speak of the 'content' as constituting a whole in a way analogous to a circle, rather than the number one. It would be strange to speak of its 'contents' unless we were going to then identify specific topics and themes or sections and so on, which would then each be discreetly distinguished things. This is how we speak of the 'content' of a book when we break it down into headings and place these in a 'table' (a setting of distinct items, placed apart from one another for the sake of clarifying the organization). To have a 'table' of 'content' would be awkward, because it would imply you are placing the continuous totality of what is contained within the book in a confined space within it. That would be crazy and doesn't make any sense. Thus, "table of contents" is proper.