At the heart of the synoptic question is the discussion on the hypothetical source document "Q."
Did it exist or is it a scholarly construct? Here are two contemporary scholars who come to completely different conclusions: John Kloppenborg (Q - Yes) and Mark Goodacre (Q - No).
At Google books, you can preview the introduction to their work and see for yourself their line of argument:
John S. Kloppenborg - Q, the Earliest Gospel
Mark S. Goodacre - The Case Against Q