Origin of Species - a long, difficult read (with a bit of an abuse of punctuation and the run-on sentence - I counted in one line no fewer than 13 commas!), but well worth it.
I have wanted for a long time to have a better understanding of evolutionary theory, and reading the beginning - Darwin's theory of Natural Selection - gave me a much better understanding of it than I even expected. So many of the "it couldn't be that way because of __________" arguments against evolution are answered here, in this book, written 150 years ago - that so few opponents ever pick up! Would people still question the holes in the fossil record if they had read Darwin's answer to that problem, already written for us to see? Would people understand the significance of similarity in embryos, or homologous bone structure if they read Darwin's writing on it? When you get into it, it is truly fascinating.
My brother-in-law James (who finished Origin far ahead of me!) quoted parts of the last chapter that are well worth reading, even if you don't have time for the whole book. As James says, there isn't much of a summary you can give that would do this work justice. I am astounded at just how much Darwin understood, even without our modern understanding of genetics. I am also amazed by just how much Darwin knew about creation and the lengths he went to to learn even more (digging through bird poop on multiple occasions and more!).
Beyond that, reading the Origin of Species makes one wonder how our society today has re-interpreted "Darwinism" to be godless. Darwin himself refers more than just once to the "Creator" in his writing. Details of his life indicate he struggled with his own religious beliefs as he was told by the church that they did not align with his theory. His writing echoes his struggle - containing a quote from "a celebrated author and divine" who saw religion and evolution as compatible. I wonder how many Christians and non-Christians alike are turned off from science (the study of Creation!), Christianity, or both as a result of being unable to harmonize God's word given in Scripture and in Nature?
Darwin looked to the future, to the young naturalists of his day, hoping they might be able to move beyond the "it's always been this way" attitude of the older generation and accept his theory. I hope something similar for the church today: that our young people might see evolution as the beautiful development of God's creation - all part of his plan - rather than as something to continue using as a wedge between the Church and the world - "for thus only can the load of prejudice by which this subject is overwhelmed be removed" - and no longer be a stumbling block for so many.