There are many well-written books that introduce evolutionary theory, at various levels. I think my favorite at the technical end of the spectrum is The Theory of Evolution by John Maynard Smith. First written in 1958, it evolved significantly over subsequent decades. My copy is the Third Edition, first published in 1975. That edition expanded previous editions with three new chapters, one of them a remarkable chapter on chromosome structure and "gene action control." It's remarkable because it includes discussion of the fact that genomes are far bigger than they "need" to be to provide structural genes, something that had been deduced by molecular biologists years before DNA sequencing had been invented. I am always impressed and humbled by the things that molecular geneticists figured out with tools that today are completely inadequate.
At the more popular-level end of the spectrum, I like The Greatest Show On Earth by Richard Dawkins, but Why Evolution Is True by Jerry Coyne is also very good. Those are recent books, both surpassed in my view by The Blind Watchmaker. Unfortunately, these authors are also loud and sometimes boneheaded critics of religion, and while those books are not anti-religious books, it would be understandable if believers wished for evolutionary biology overviews without the baggage. Perhaps others will chime in here to provide recommendations.
For a more compendium-like approach to acquiring overviews of evolutionary science, I recommend the series of special issues of PNAS, published over several years as I recall, called "In Light of Evolution." I think there were at least 4 of these collections, all open access. They should probably be in a collection of links somewhere here on BL, and perhaps they already are.