The subtitle claims that the book deals with "The Classic Texts". While we can busy ourselves splitting hairs about precise definitions, the word "classic" often deals with anything pertaining to the ancient Greek or Roman world. Perhaps "ancient" is the key word here. Or to be generous we can settle on the word "old".
When a writer or editor claims to be dealing with "classic texts" I, at the very least, expect to find documents dating back to the time of the ancient Greeks and Romans. I want to read old things. When I opened Jesus Beyond Christianity what I found was a mix of new and old literature. Among the new documents were selections from 20th century writers such as Muslim scholar Ghulam Ahmad Parwez and the current Dalai Lama. Selections by Jewish, Muslim, Hindu, and Buddhist scholars were written some time within the past two hundred years.
If the editors of Jesus Beyond Christianity really want to consult classic texts they could have focused on ancient writers such as Tacitus and Suetonius. They could have consulted the Babylonian Talmud. There are scores of possibilities they could have used.
Barker and Gregg have edited a very interesting book but it doesn't hold its focus on the so-called classic texts. If they were true to the content of their collection they could have used a phrase like "Historic Texts" in the book's subtitle.
So we have to be careful when purchasing or borrowing a book. There is sometimes a difference between what's written on the cover and what's found inside. The difference between the two can be significant.