If you've ever wanted a good, short and informative defense of the Trinity, then turn to the chapter on "LORD" in 18 Words. In just 11 pages Packer does a good job of verbalizing the truth of the Trinity from Scripture and the reason it is important.
He traces the use of the word Lord through the Old and New Testaments and shows, convincingly, that Trinitarian theology was not developed 400 years after the Scriptures were written, but was known and believed by the very writers of the New Testament.
Specifically interesting to me was his defense of the Holy Spirit's distinction as a "he" and not an "it." I think he is right to say that understanding the relationship between the Father, Son and Spirit are essential to understanding the Gospel.
I end with a quote from the last page of the chapter:
To put it the other way round: the gospel says that there was in God from eternity mutuality of love and joy (John 1:1f.; 17:5,24); that men were made to share this fellowship; that when sin had made this impossible, God came in person--the second Person, sent by the first Person and empowered by the third Person--to save us; that God-made-flesh died for us, lives for us, united us to Himself, brings us to God the Father now and will take us one day to share His glory; that a divine Guest, the Holy Spirit, indwells each Christian (there are over 800 million of us alive today, leaving aside the faithful departed) to prompt prayer and transform our fallen nature; and that Jesus Christ is companion and friend to every single believer, giving him or her constant and undistracted attention. It is surely obvious that one of these marvelous, almost fantastic things could be said save on the supposition that Father, Son and Holy Spirit are God--in other words, that God is Father, Son and Holy spirit. Those who deny the Trinity have to scale down the gospel--and do.
And so, I leave you with the Gospel in hopes that you spend the night thinking, talking and believing in Christ the Lord and Savior, so that perhaps you can spend the rest of the year doing so, as well.
-joe