I'd say the Bible is capitalised because it's *the* bible, the only book that mattered (supposedly). But if you take the word 'bible' as in "the bible for programming in the computer programming language Java", I wouldn't say it should be capitalised, as even though clearly it is saying that the book should be the first reference for authority, similar to the "Holy Bible", it is not THE Bible.
"Bibles" is an odd word, I don't know whether I would capitalise it. I suppose that the idea of referring to more than one copy of a Bible is odd personally - I don't sell them, or lend them, and I'm not a priest who would probably have lots of them lying around (and I generally don't use one myself, but that's beside the point). But I could imagine that sense of the word being capitalised, just not used very much, as I don't know very many people with one Bible, never mind several
But capitalising "Bibles" in the sense that there is more than one version of the Bible is extremely strange to me - I know that there can be many versions of the Bible due to differences in translations and indeed beliefs, but to refer to different versions of the Bible makes it lose its uniqueness and authority, a bit like how monotheistic folk talk about [one] God, but [false] gods. So imo it's a tricky word, due mainly to varying religious influences.