This proves, as if it needed proving, that the originators of profound ideas often given the clearest, most readable, and most complete discussions of their topics. Explainers often just muddy the issue, and most later researchers incrementally widen, fill in, and bolster the original points. If any intelligent reader wants to understand the mechanism, breadth, subtlety, and power of evolution, this is the place to start. If nothing else, Darwin gives clear statement (and rebuttal) to issues that biblical literalists still yammer about, including the time scale of speciation, the fragmentary nature of the fossil record, and the fallacy of 'irreducible complexity.'
"Slow though the process may be, ... I can see no limit ... to the beauty and infinite complexity of the coadaptations between all organic beings" Understanding doesn't dampen awe. Quite the opposite: truly appreciating the power of change and selection conveys a majestic sense of the world and our place in it that I can not express. And, although I'm not a theist, I can certainly see how the the limitless power of never-ending creation can be seen as a direct and present act of a limitless Creator.
Only a very few things will sound unfamiliar to the modern reader. The first is the absence of genetics, from Mendel to Watson and Crick. Darwin observed and described inheritance without any sharp statement of what was inherited - genetics provides the mortar between the stones of Darwin's edifice. Another is the creationist beliefs of his time: that each "species" was a distinct act of creation, and progenitor of the many extant subspecies and varieties. Yet another is his unwillingness to believe that "any part of the structure of any one species had been formed for the exclusive good of another species." Mutualistic coevolution is real: a flower's nectar is of no direct use to the flower, but serves the insects around it. In a wider sense, though, nectar indirectly benefits the flower by attracting pollinators, so the error may lie only in too tight an interpretation of "exclusive good."
This slim book has been edited down from a much longer work, and I do not know what was sacrificed to brevity. Still, it stands well by itself, and the short distance from front cover to back should appeal to people put off by thick books. I recommend this to every thinking reader, down to high school age or earlier.