For general philosophical background, I would recommend the works of Eugen Rosentstock-Huessy and Dietrich Bonhoeffer. Though not necessarily libertarian, their views generally promote liberty and progress from a scholarly or educated Christian viewpoint.

“The Law” ~ by: Frederic Bastiat
Read it free online here:  http://christianwarmongers.com/?page_id=61

“Preface

When a reviewer wishes to give special recognition to a book, he
predicts that it will still be read “a hundred years from now.” The Law,
first published as a pamphlet in June, 1850, is already more than a
hundred years old. And because its truths are eternal, it will still be
read when another century has passed.

Frederic Bastiat (1801-1850) was a French economist, statesman, and
author. He did most of his writing during the years just before – and
immediately following — the Revolution of February 1848. This was the
period when France was rapidly turning to complete socialism. As a
Deputy to the Legislative Assembly, Mr. Bastiat was studying and
explaining each socialist fallacy as it appeared. And he explained how
socialism must inevitably degenerate into communism. But most of his
countrymen chose to ignore his logic.

The Law is here presented again because the same situation exists in
America today as in the France of 1848. The same socialist-communist
ideas and plans that were then adopted in France are now sweeping
America. The explanations and arguments then advanced against socialism
by Mr. Bastiat are — word for word — equally valid today. His ideas
deserve a serious hearing.”

Jesus for President: Politics for Ordinary Radicals – Shane Claiborn and Chris Haw
Justice: Rights and Wrongs - Nicholas Wolterstorff 

Jesus for President presents a “for beginners” survey of the Biblical story and highlights its political undertones. It presents the salvation story as a struggle between God’s people and evils of Empire. The authors write about how Christians are meant to be a peculiar people, set apart, in-but-not-of the world. They present faith in Christ as political allegiance to a different King and a different Kingdom than any in the world, and the Christians’ mission as showing the world that a better world is possible under Christ’s Kingship by being ourselves a foretaste of that better world. Informative, readable, and inspiring. 

Justice sets out to develop a working theory of justice and rights. The author engages with many philosophical theories about justice and rights and develops his own with tedious rigor. It is a dense but rewarding read, and it has informed a lot of the language I use when I discuss issues of liberty, justice, and rights with people.