The Bible is foremost. For a description, see The Bible.
The following (in alphabetical order) contain insights that have helped me clarify my views, but none includes an adequate statement of real Christianity. (Outside the Bible and my own work I am not aware of such a statement.) As to whether their authors were Christians, my answers range from "highly probable" to "clearly not".
John Barrow and Frank Tipler, The Anthropic Cosmological Principle (c.1990) is long, some of it is wild speculation, and only a small part of it is relevant here. But it shows, in more detail than any other source I have seen, that with the standard assumptions about the state of the Earth a few billion years ago the probability that chemical combinations as complex as our organic ones would emerge is very small. (Sadly the authors avoid the obvious conclusion and take refuge in the parallel worlds theory.)
Herman Dooyeweerd, A New Critique of Theoretical Thought (c.1935) is the most comprehensively penetrating elaboration of a Christian worldview ever written.
Herman Dooyeweerd, Roots of Western Culture (c.1950?) is shorter than A New Critique and more accessible than In the Twilight of Western Thought which is sometimes described as "Dooyeweerd's introduction to his philosophy".
Charles Hartshorne, The Divine Relativity (c.1950) is the classic account of how we need to see God, not as statically predetermined, but as constantly, creatively responsive. It builds on some ideas from Part 5 of Whitehead's Process and Reality.
Peter Hitchens, The Abolition of Britain (c.2000) shows how in recent decades Humanism (which Hitchens calls "cultural Marxism") has transformed British society. (Hitchens prescribes statism, but his diagnosis, which occupies most of the book, is accurate.)
C.G. Jung, The Relations Between the Ego and the Unconscious (1928) is valuable as a corrective to the currently prevailing expectation that men and women be almost entirely interchangeable in their roles. I have singled out the cited work as perhaps the earliest in which Jung's fully mature position is expressed, but several others would probably serve equally well. But beware of his popular Answer To Job, which merely elaborates his erroneous identification of the antithesis between good and evil (which is irreconcilable) with the polarity between dominant and subordinate mental functions (which is reconciled in individuation). Anthony Stevens, Archetype (1982), is an excellent restatement and updating of Jung's thinking, and my only complaint is that he succumbs to the fashionable celebration of homoerasty, in violation of his own previous analysis.
C.S. Lewis, The Abolition of Man explains with overwhelming clarity the absurdity of the prevailing Empiricism.
C.S. Lewis, Broadcast Talks (1942), later incorporated in Mere Christianity draws attention to the universal sense that Something Is Wrong, and shows how the classic Christian doctrines account for and resolve this.
Konrad Z. Lorenz, King Solomon's Ring (1950). The founder of sociobiology shows us through his geese that though more than beasts we are, as Lewis used to say, not less.
John Lucas, The Freedom of the Will (1970) pursues the arguments about physicalism in great detail, expressing many important insights clearly and succinctly. A short version of his argument is freely available online as Minds, Machines and Godel (1959).
Ludwig von Mises, Human Action (1949) explains the logical (a priori) consequences of the insight that selves make choices. I use the word "economics" differently, but that is a point of clarity, not of substance.
John Norman, Nomads of Gor (1969) is one of the Gor novels, which contain, embedded in an often tiresome setting of heroic science fiction, what as far as I know is the most thorough extant fictional exploration of a thoroughly androcratic point of view. (If there are others pray let me know, because Norman is not a good writer, and some Gorean values are more Nietzschean than Christian.) Nomads was the first to begin this exploration in earnest, and is probably still the best novel of the series. "The institution of female slavery" has been further elaborated since then, but not in any fundamental way as far as I know (though I have not read the entire series).
Olthius and others, Will All the King's Men (1972) is a group of essays describing in general terms what kind of approach to life in the world is appropriate in modern circumstances. It explains that there is far more to Christ's Ekklesia than the limited organisations usually called churches. I found Bernard Zylstra's contribution, Thy Word Our Life, especially stimulating.
Talcott Parsons, Working Papers in the Theory of Action (1953) is by far the most impressive attempt that I know of at a theory of human action that aspires to predict the variety of types of human action from a few first principles. Most of Parsons' later texts are applications to various areas of the theory first elucidated here. One of his last essays, A Paradigm of the Human Condition (1977), tries to place his thinking in the broadest possible context. To my mind he tries to start from too rarefied a set of axioms, and of course he fails to distinguish the original structure of the world from the consequences of Defection, but I suspect that there is more to learn from his attempt than I have yet absorbed.
Karl Popper, The Logic of Scientific Discovery (1934) demolishes the Verification Principle, showing that none of our scientific theories have any substantial probability of being true, undermining the prevailing dogmatically scientistic understanding of life. (This left Popper himself in a kind of nihilism because he sees hypotheses as arising at random, but it is fair for us to use his breakthrough to support a more positive doctrine.)
Murray Rothbard, For a New Liberty (c.1980) is the founding text of anarcho-capitalism.
H. Evan Runner, The Relation of the Bible to Learning (written 1959/60) applies Dooyeweerd's thinking to North America, explaining that the whole of life is "religion" (i.e. Attitude), focusing here on the academic area, explaining the importance of separate Christian academic organisation.
H. Evan Runner, Scriptural Religion and Political Task (written 1961) continues the "life is religion" theme, here focusing on the political area, explaining the importance of separate Christian political organisation.
Francis Schaeffer, The God who is there (1968) is included because of its treatment of what Schaeffer calls "connotation words", to which I am much indebted. Beware, however, of certain rationalistic assumptions of which Schaeffer, in his justified horror of existentialism disguised as Christianity, has failed to steer clear.
Herbert Spencer, The Man versus The State (1884) espouses a society based on free collaboration, and describes how the Liberal Party has adopted the essence of Toryism, forced collaboration.
Herbert Spencer, First Principles (1851) expounds the role of vibration, while remaining steadfastly blind to much else.
Thomas Szasz, The Myth of Mental Illness (1960) explains how the concept of mental illness undermines responsibililty and justifies oppression.
A. N. Whitehead, Process and Reality (1929) is the classic text of "Process" (or "Organic") ontology. It is inadequate because it neglects the union of the relative and the absolute and leaves no room for real human responsibility and Defection.