When "sum" signifies existence, shouldn't it most often come at the start of the sentence?
Just an educated guess, but I think the verb comes (or rather, appears to come) at the beginning of the sentence in English only because we use a dummy demonstrative ("there") for existential phrases like this. (German has a dummy subject in "es gibt x", where the "es" doesn't really signify anything.)
Since the Latin prase contains no "filler material" like that, I don't think the word order needs to change at all, as change in word order adds focus to a particular part of the sentence. You'd probably end up saying something like "Oh, but there are many books!"
That's true with much of Latin, but when it comes to "sum" doing what English/German use an grammatical expletive for, it actually does come at the beginning of the sentence most often. That's because normally sum--or to be, or sein--signify an equality of sorts. But when it functions as an expletive, it insists on the existence of the thing.
Britannia insula est. Britain is an island. 'Britain' and 'island' are equivalent terms and that is the purpose of the sentence. 'sum' here functions as a copula.
Est insula Britannia... There is an island, Britain... Here it is not the equivalence, but the existence that is emphasized. The two sentences have not the subtle differences that most word order variations give in Latin, but 'sum' actually functions different.
In this Latin sentence, "There are many books" is a way to translate what would otherwise be "The books are many" simply because the English is too awkward in the latter.
(I'm synthesizing this from Sidwell & Jones's "Reading Latin" and Keller & Russell's "Learn to Read Latin")
Wow, thank you for explaining so well and so patiently. I am really sorry I tried to "explain away" your relevant and informed question with my irrelevant speculation. :)
(And I love "Est insula Britannia..."! Can someone please write me a fairy tale starting like that? :D)