I tend to think Emerson is overstating his point, but as I understand it, he means that people gain knowledge from books, and though that is a good thing, it means they can also misread an author's message and thus get the wrong ideas from a book. A writer's theme or intended purpose can be twisted by a reader into something else or interpreted wrongly based on a preconceived notion a reader already has.
If we look at Emerson himself, "Concord Hymn," is a case in point. This, to me, is a moving poem because it expresses the naive but true spirit of America's founding, and its ideal. The lines,
By the rude bridge that arched the flood,
Their flag to April's breeze unfurled,
Where once the embattled farmer stood
And fired the shot heard round the world.
have themselves gone around the world as a memorial for the defiance of tyranny and the ability of one man to foment a change in the system, or at least to start a change. But couldn't the poem also be unthinkingly read as a celebration of violence and war in general? Even today, there are those who might use Emerson's words as a signal for us to go elsewhere in the world, when it is none of our business to do so, and cause violent change.
Melville and Douglass, like any other authors, can be misread and misinterpreted as well. Suppose, for instance, someone took away from Bartleby the notion that the sort of withdrawal from life Melville describes is the way we should all act? Admittedly this is an extreme example, but in a more subtle fashion, some readers today might judge the treatment of Bartleby by others in the story as a kind of return to the supposed good old days when a mentally disturbed person was allowed to deteriorate, as in this nineteenth-century scenario, it was not unusual to allow to happen. This would surely be an "abuse" of what should be a timeless story.
With regard to Douglass's autobiography, the statements in it about religion could easily be misinterpreted even today, unfortunately. Douglass, in the Appendix to his Life, made it clear he was not attacking religion per se but rather the way it was being misappropriated by slave owners. Yet bigoted people at that time (and I wouldn't be surprised to find many such people today, though they probably would not be reading Douglass in the first place) could and did use this apparent freethinking stance of Douglass to discredit his entire message against the slavery system.
As Emerson's quote indicates, books can be interpreted for good or for ill. We can only hope for the best.