There's a beautiful line early in Herder's "Sculpture" that sums up most of his argument--"Sight gives us dreams, touch gives us truth."
Herder focuses in this book on the merits of sculpture and how our (the viewers) experience of sculpture differs from our experiences with painting and poetry. In the first half he starts from the metaphor of a blind man whose sight was restored to explore how we learn to use our senses to experience the world around us, and how sight/vision becomes a shorthand for so many other senses that--the flower we see and assume smells good, the food we see and assume tastes good, etc. Sight especially becomes a shorthand for touch, and paired with imagination it allows us to look at well-done sculptures and see that they could be human. Instead of touching them with our hands, we become able to touch them with our souls, and in that way they go beyond mere visual art and become truth.
The first three chapters were written and then a large gap of time happens before the last two, which makes them feel slightly off-balanced. Herder's ideas changed somewhat, but he apparently did not go back and edit them much, so there's a bit of tension between the first half of the book and the second, which makes it more interesting.
This particular edition is also accompanied by a great introduction by Jason Gaiger which is worth perusing in it's entirety, as he sums up Herder's life and philosophy well.