In his new book Rethinking the Trinity and Religious Pluralism: An Augustinian Assessment (IVP, 2011), Keith Johnson writes: “Because the Trinity represents a presupposition of the gospel, distortions of this doctrine inevitably lead to a distorted gospel.”
Fred Sanders, in his theologically insightful and winsomely practical book The Deep Things of God: How the Trinity Changes Everything (Crossway, 2010), argues that “the gospel is Trinitarian, and the Trinity is the gospel.”
(I’m not sure how long the sale lasts, but Amazon has the Kindle eBook version of Sanders’s book on sale for $2.99, which is 83% off.)