By G. K. Chesterton.
This is my second time trying to read this book. This time I actually finished, although I am sure that in rereading it (eventually) I would glean more.
The sum of Chesterton's argument is encapsulated in the several page "SUMMARY OF THIS BOOK" which appears at the end; it might not be a bad idea to read this first. He contends that man and the Church are both things unique, strikingly so when considered on a level with other things purportedly of their kind. If man is an animal, he is the only animal capable of claiming so; if the Church is a mythology or philosophy, it is the only one which unites the intellect and the spirit of romance, not to mention unabashed hope for things to come. (Chesterton makes a point of distinguishing between pessimism and optimism, which are to him types of fatalism, and hope, which allows for free will.) It is the only with a Gospel, good news that must be spread. It is, as he says, the Church Militant, of which Islam is a later and paler imitator.
Many of his arguments are surprisingly simple, but that does not mean you do not have to read carefully: they may be gone before you realize he is making them. One of the striking ones in my memory is that the doctrine that God is Love and the doctrine of the trinity are nearly the same thing; Chesterton says:
For if there be a being without beginning, existing before all things, was He loving when there was nothing to be loved? If through that unthinkable eternity He is lonely, what is the meaning of saying He is love? The only justification of such a mystery is the mystical conception that in His own nature there was something analogous to self-expression; something of what begets and beholds what it has begotten.
Certainly this book is a weighty slog; it is probably unreasonable on my part to expect light reading on weighty matters. But there are also thought-provoking nuggets together with some humour (for instance in the obversation that no mortal power can prevent the poet from contemplating the skylark in spring). Is this book worth reading? Probably, but only if you take it seriously.
An addendum: I read the Ignatius Press reprint, but this book and many others by Chesterton are in the public domain and available online.