An enormously harmful book by any standards is The Protocols of the Elders of Zion. How many deaths might it have caused over the last 100 years, in Russia as well as Germany, and how much turmoil is it even now responsible for among fringe groups convinced of its validity despite its first exposure as a malicious fraud coming in 1921. I have it as an appendix in a fairly lunatic conspiracy theory volume 'Behold A Pale Horse' in which the author William Cooper suggests we replace 'Jews' with 'Illuminati'! It's a very effective tool for evoking a paranoiac mindset, an indisputably powerful and sinister text.
And if we are to judge harmful books by the numbers of real-life deaths caused then the all-time winners must be the Bible and the Koran. I don't know the Koran well enough to offer much of a comment on its virtues, but my favourite Old Testament books are Ecclesiastes, the Song of Solomon, most of Psalms and Proverbs. Psalm 88 is a kind of power electronics lyric, wonderfully bleak and nihilistic. The KJV is a great pleasure for any reader with English as a first language, so many of our everyday expressions are taken from it: the influence on the language has affected our way of perceiving the world. Taken as a whole the Bible is surely a dangerous book with very great potential to fuck up lives.