It is not a book, of course, but a short letter. It quotes a Hidden Word, No. 1 of the Arabic (but with the grammatical difference that the plural imperative is used, whereas in the text of the Hidden Words we now have the grammar is singular). My guess is therefore that it was written around 1857 shortly before Bahá'u'lláh put the Hidden Words into final shape.
This work is the clearest indication I know of Bahá'u'lláh's self-conception before about 1859, when he appears to have begun telling people like Fitnih and Nabil-i Akbar that he was the promised one. Denis MacEoin pointed out in his 1989 BRISMES article that Bahá'u'lláh in this work disclaims having any "Cause" at that point, and my rereading it now in conjunction with my translation convinces me that Denis is right. He has no "iqbal bar amri," is making no claim to have a divine Cause.
This work gives us a humanist Bahá'u'lláh, who sternly denies being able to work any miracles, who defers humbly to the Mirrors of the Babi dispensation, who gives us a catechism that includes belief in God, the Bab, Quddus, and the "Living Countenance" (Denis thinks this is Azal; I don't know Babi terminology well enough to have an opinion). Indeed, the argument seems to be made that just as plagues no longer break out in Iraq every 30 years as they had in past centuries (owing to Ottoman quarantines, by the way), that after the Bab's death the age of miracles is over with. This is in turn an announcement of a profound secularization of sorts, isn't it?
This brief letter seems to me proof that Bahá'u'lláh's "messianic secret" (for which I have argued) probably should not be dated further back than about 1859, from which time we begin getting independent eyewitness accounts of his having privately put forth a claim. In short, it raises the most acute questions about the nature of the "intimation" Bahá'u'lláh is said to have experienced in the Siyah Chal. If one reads the account in Epistle to the Son of the Wolf carefully, it appears that it consisted more of ilham or inspiration than of wahy or revelation, and that Bahá'u'lláh began thinking of islah or reform of Babism rather than of making any claim of his own. If in fact the Book of the Tigris post-dates the poetry of the Sulaymaniyyah period, I probably should retract my messianic reading of the Ode of the Dove in favor of seeing it as an example of Sufi effusion or ecstatic enthusiasm (shath).
On the other hand, Bahá'u'lláh is after all in this letter speaking rather authoritatively and handing out spiritual advice. If the title "Sahifih" goes back to the Baghdad period then he is using a word normally employed for scripture. To put it bluntly, who does he think he is? A sort of Babi Sufi shaykh? A manifestation of the attributes of Imam Husayn alongside other Babi manifestations? What is clear is that his self-conception changed mightily between the early 1850s and the later 1850s.