Once upon a time, before there were gospels of the kind familiar to readers of the New Testament, the first followers of Jesus wrote another kind of book. Instead of telling a dramatic story about Jesus' life, their book contained only his teachings. They lived with these teachings ringing in their ears and thought of Jesus as the founder of their movement. But their focus was not on the person of Jesus or his life and destiny. They were engrossed with the social program that was called for by his teachings. Thus their book was not a gospel of the Christian kind, namely a narrative of the life of Jesus as the Christ. Rather it was a gospel of Jesus' sayings, a "sayings gospel." His first followers arranged these sayings in a way that offered instruction for living creatively in the midst of a most confusing time, and their book served them well as a handbook and guide for most of the first Christian century.It's just so a matter of fact. The once upon a time, this is what really happened line. Always makes for a good book!
Then the book was lost. Perhaps the circumstances changed, or the people changed, or their memories and imagination of Jesus changed. In any case, the book was lost to history somewhere in the course of the late first century when stories of Jesus' life began to be written and became the more popular form of charter document for early Christian circles. (Burton L. Mack, The Lost Gospel: The Book of Q and Christian Origins.)
In related news, on the Cynic thesis I have finally started with F. Gerald Downing (see books such as Cynics and Christian Origins). I must say, he is far more convincing a proponent than Crossan and Mack!