Paul's letter to the Ephesians was very likely not written by Paul, but it's to him this book is attributed.
As per the previous letters from Paul, this epistle is all about telling the recipients what's what in terms of Christian living.
1. Be humble, gentle, and patient with each other (4:2)
2. Quit lying and start telling the truth (4:25)
3. Work for a living instead of stealing (4:28)
4. Replace bitterness, wrath, brawling, slander, and all malice with kindness and compassion (4:31 - 32)
There's more, but that's a sample. One of the main messages is that all benefits we receive in this life are by the grace of God, not earned or deserved, but simply given to us out of His love.
"For by grace are ye saved through faith; and that not of yourselves; it is the gift of God: Not of works, lest any man should boast. For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them." -- Ephesians 2:8-10
This book also highlights the analogy of Jesus being the cornerstone of the church, and how people in different relationships should treat each other. For example, wives should obey their husbands, and husbands should love their wives as Jesus loves them, and slaves should obey their masters while masters should treat their slaves with respect. We have to remember the context and time in which this letter was written, of course, because Paul isn't saying slavery is a good thing, or that women should be subjugated by men; he was simply giving advice based on what was the current reality of his day and his time.