I have used the following books to prepare the series of expositions on Philippians:

• The Message of Philippians by Alec Motyer.
• The Daily Study Bible. The Letters to the Philippians.. by William Barclay.
• Tyndale New Testament Commentaries. Philippians by Ralph P. Martin.
• A Bible Commentary for Laymen. Philippians. Bound for Joy by Stuart Briscoe
• Philippians. Be Joyful by Warren W. Wiersbe.
• The old Pulpit Commentary on Philippians.

I found the books by Briscoe and Wiersbe particularly helpful and enjoyable to read.

Paul's epistle to the Philippians is a happy, confident, letter in spite of Paul's situation being difficult and the problem of disunity in the church to which he wrote. I think one of the reasons for the optimistic, cheerful and triumphant tone is because it is a, 'thank you' letter. Paul was much encouraged by the concern shown by the Philippians for his welfare and the gifts that they sent him. Paul says in his epistle that he has learned to be content whatever state he finds himself. However, even the great apostle found it easier to be happy after encouragement and appreciation. The tone of his second letter to Timothy is very different from this one to the church at Philippi. I do not think Paul's mood could be entirely divorced from his circumstances. This might be of some comfort to those of us who find it difficult to live up to apostle's confident assertions in this his most winsome of epistles.