Latour's We Have Never Been Modern kind of spoils the plot in the title, but basically, the thrust of the work is exactly that: that we never really have been modern. By modern, he doesn't mean current or contemporary, but rather, he expresses modernism as the belief that current time is essentially different than previous eras.
For Latour, the entire premise of modernism, and therefore of its subsequent belief, post-modernism, that basically, small cultural divisions were responsible for the belief that we are different because of the scientific method. You could summarize his argument this way: We are no less human than our ancestors, and our ideas are not more objective just because they rely on the scientific method.
This argument, as Latour understands it, goes all the way back to Plato, the first philosopher to really analyze the nature of objectivity and subjectivity. Basically, Latour uses Plato's arguments around perception to disprove the cultural assumptions that the enlightened, scientific cultures of the west tend to make. Just because we are using data sets which we agree on doesn't mean we're thinking the same thing at all, and if you wanted to know what being a human is really like, any human story on the earth might hold something significant, regardless of whether the writer of the story believed in God or not.
In other words, Latour argues that we made a mistake by agreeing to prefer our own world as inherently more valuable than other cultures, simply because they seem more primitive for their religious or cultural views.
