If the Book was intended from the start to be a mystical text, why was it not more obvious about this feature?

I suspect that ethical awareness is relatively new in terms of our evolutionary history as a species. In the beginning, there may have been immortal amoral prophets that could have felt threatened by the prospect of the evolution of fairness and impersonal cosmic law. To prevent them from enclosing justice before it began, they could have been given the opportunity to project superstition and a magic show wherein the socially absorbed text made the general populous easier to control. 

The Book provides humanity a fair totem in God, even if God is at first understood to be something superstitious which does not actually exist. Because of the monotheistic (and other spiritual) traditions, even non believers socially absorb from the cultural majority a moral instinct toward fairness. Not necessarily a very strong one - but one that commonly exists in some form all over the planet. 

As an example, there are places where the Quran seems to condone slavery: Surah 23:1-6, it seems like Muslims are to be rewarded for guarding their chastity, but are free from blame if they take advantage of their female slaves. These verses could also be interpreted to mean that the slaves are free from blame for violation of chastity. Yet slavery is so ancient, and the slave owners traditionally so powerful, that this seemingly inexcusable phrasing might have been necessary just to eventually enclose slavery at all. 

Verse 33:50 of the Quran: "Prophet, We have made lawful to you the wives to whom you have granted dowries and the slave girls whom God has given you as booty." Quite plausibly, God never provides slave girls as booty. The verse would not be contradicted if this were the case.

I suspect that the Book of Revelation reveals the rest of the text because so dense with symbolism, digging for the esoteric interpretations elsewhere, where always less difficult, becomes easy.

The Quran Surah Al-Ma'idah (5:45): "We ordained for them in it: 'A life for a life, an eye for an eye, a nose for a nose, an ear for an ear, a tooth for a tooth, and for wounds equal retaliation.'"

Eye for an eye provides a system of measurement the verses must stand up to, so that nothing can be advocated which violates eye for an eye.

It is not a lie, nor an attempt to deceive, to phrase the text in a manner to avoid getting caught, to avoid leaking crucial military data at the wrong moment - even if there is (unbaited) opportunity to project something else.