"Delight yourself in the Lord." is a verse from the Old Testament book of Psalms (37:4) that represents a command from the Word of God. It means that we should pursue joy in God. The Bible makes it clear that God created humanity to rejoice him, to make it our central purpose. Thus it stands to reason that the more passionately we pursue our purpose, i.e. delighting ourselves in the Lord, the more we will glorify the Lord and the more we will fulfill what God has designed us for. Such is the basic tenet of the book Desiring God by John Piper.
The book has chosen to make its arguments in the areas of worship, love, money, marriage, missions, and suffering from the standpoint of what Piper is urging us all to become, that is a "Christian hedonist". Of course, many folks are very uncomfortable with the term hedonist. A hedonist is someone whose life is devoted to seeking self-pleasure by whatever measures and means are necessary. Hedonism is the doctrine that the pursuit of pleasure is the highest good. I am sure that Piper chose this terminology purposefully to make folks uncomfortable and uneasy, to be provocative. To make them shift in their seats and shake their heads. The term hedonist is unequivocally linked with gluttony. All of these images have strong negative connotations.
Yet contrary to how we think about this label, Piper implores us that unless we approach God as a hedonist, then we aren't truly worshipping Him and giving Him what he desires and commands of us. If he could boil down his entire message in a single sentence it would be that "the chief end of man is to glorify God by enjoying him forever". The more zeal and passion that we devote to this task, the more we will glorify God. Such is the notion of being a Christian hedonist.
The Bible plainly teaches that the goal of all we do should be to glorify God. But it also teaches that in all we do we should also pursue the fullness of our joy. Some theologians have tried to force these two pursuits apart. But the Bible does not force us to choose between God's glory and our joy. Many people mistakenly feel that if they experience joy or enjoyment or pleasant feelings in their work for the Lord, then they are not doing the work for the right reasons. Piper would argue to the contrary and he backs it up with fastidious attention to what the Bible actually says. So, go ahead, live hedonistically for the Lord, just make sure that you really understand what this means. Oh, and don't dive into this book unless you are willing to deal with a fair level of complexity and bloviation for a few relatively straightforward nuggets.