I guess "had been written", "has been written" and "was written" should be acceptable.
You have to consider the tenses of both esti and the participle here. 'Estis' and 'skribita' both express past, hence 'had been'. If I'm not mistaken, 'was written' would be 'estis skribata' (past/present). 'Estis skribota' (past/future) would then be something along the lines of 'was to be written' (I think). They're all in the past from the point of view of the speaker (esti). However, the relation between the writing of the book and the past is different (participle): skribita (further past), skribata (the moment), skribota (to the future).
That's the best way I can explain it, but I am still learning myself. :_D
(Btw, with the same logic, 'has been written' would be 'estas skribita' (present/past): time spoken about is present and writing of the book is in the past.)
There have been huge debates about how to interpret participles such as -ita, but I think the side that eventually prevailed is that -ita etc. do not indicate tense but only aspect.
So -ita indicates that something is completed and -ata indicates ongoing action.
Thus, "La libro estos skribita morgaŭ" could mean either "The book will be written (and finished) tomorrow" or "The book will have been written tomorrow", while "estis skribata" is more like "was being written".
"Estis skribota" would be "was to be written", as you said.
Ah, yes, that is a much clearer way of explaining it. I rewrote my comment at least thrice but I wasn't really happy with any of the versions. :_D