There is something somewhat misleading about the title of this book - in fact, most of the selections contained in this book are not 'lost' documents at all. For example, the Dead Sea Scrolls (which are not contained in this book) were well and truly lost - they were buried for centuries, and those who buried them did not leave notice of where they were in any way that survived; hence, when they were found, they were truly recovered from having been lost. What makes the documents in this book 'lost' contextually is that they were not included in the canon of the New Testament texts.
In the early church, there was no consistent agreement about what belonged in the Bible and what was not to be included. Modern-day readers of books such as the 'Da Vinci Code' can understand some of the issues and dilemmas here - there were no printing presses, no consistency of library cataloging or copyrighting, and no central authority in the oft-persecuted church as to who could make a decision of what texts were valuable. Indeed, even the Hebrew scriptures existed in variations, and the modern day issue of apocryphal books included or not included in the Bible stems from the difference between the Greek and Hebrew translations of the Hebrew scriptures/Old Testament texts.
For example, the book of the Shepherd of Hermas, included here, was well known to the early church, and continued to be a document known to church historians and scholars in the subsequent centuries, even if it was not commonly known and read by the laity in the churches. The apostle Paul was far from the only letter writer of the early church; the New Testament includes some other letters (Peter, John, etc.), but the canonical decision at one point was made that only those attributed to the original twelve apostles and Paul would be included - the letters of Clement, very important and useful in the early church, were excluded from the canon.
However, there is an important point to be made here. Just because something was not included in the canon of scripture did not mean that it was worthless. Just because a student gets an A rather than an A+ does not mean that student did not do good and worthwhile work. Many of these texts were 'A' texts for the early church, but lacked that certain something that made them canonical. The same is true for the gospels - this collection includes some extra-canonical gospel texts, and the reason for their non-inclusion in the canon is varied, but does not mean their value is insignificant.
There has been much work done on early church documentary history and the development of the canon since this book was first published more than a century ago, but these are reasonable (if not always reader-friendly) translations of relatively unknown texts, useful for general readers and undergraduates, or those who might want to know what the early Christians were reading and being influenced by apart from the canonical text.
