BDAG

A Greek-English Lexicon of the New Testament and Other Early Christian Literature by Walter Bauer and Frederick William Danker, (Third Edition; based on previous English editions by W.F. Arndt, F.W. Gingrich, and F.W. Danker; University of Chicago Press, 1957, 1979, 2000). ISBN 0226039331. The modern standard NT Greek lexicon.

Fee

Gordon D. Fee, Paul's Letter to the Philippians (New International Commentary on the New Testament; Eerdmans, 1995), ISBN 0802825117. Excellent, quite detailed.

ISBE

The International Standard Bible Encyclopedia, Geoffrey W. Bromiley (general editor), (Eerdmans, 1979-1988; fully revised from the 1915 edition). ISBN: 0802837859. Excellent 4-volume scholarly, evangelical Bible dictionary.

Lightfoot

J.B. Lightfoot, St. Paul's Epistle to the Philippians (Zondervan, 1953; reprint from original Macmillan, 1913 edition; Croosway, 1994 reprint), ISBN 0891078002. Classic commentary, treats the Greek text.

Martin

Ralph P. Martin, The Epistle of Paul to the Philippians: An Introduction and Commentary (Tyndale New Testament Commentaries; Eerdmans, 1987), ISBN 0802803105. The low-priced leader for an up-to-date, careful commentary on Philippians from a leading evangelical scholar.

NIDNTT

New International Dictionary of New Testament Theology, Colin Brown (editor; Zondervan, 1975-1978; translated with additions and revisons from Theologisches Begriffslexikon zum Neuen Testament, Coenen, Beyreuther, and Bitenhard, editors), ISBN 0310332389. 

O'Brien

Peter T. O'Brien, The Epistle to the Philippians: A Commentary on the Greek Text (New International Greek Testament Commentary; Eerdmans, 1991), ISBN 0802823920. Excellent, covers the Greek text.

TDNT

Theological Dictionary of the New Testament, Gerhard Kittel and Gerhard Friedrich (editors), Geoffrey W. Bromiley (translator and editor), (Eerdmans, 1964-1976; translated from Theologisches Wörterbuch zum Neuen Testament, ten volume edition). CD-ROM, ISBN 1577990994. Standard for many years, mixture of conservative and liberal treatments of NT Greek words.

Vincent

Marvin R. Vincent, A Critical and Exegetical Commentary on the Epistles to the Philippians and to Philemon (International Critical Commentary; Edinburgh: T.&T. Clark, 1897, reprinted 1968, 1987). ISBN 9997765257. An old classic.