Below is a good primer that gives an overview of the New Testament textual origins and defends the quality of its transmission to us today. It is short and concise but I think still delivers an in depth quality. It is written by Christians, and so will have that "bias". If you are looking for criticisms on the NT, there are numerous other books out there. But for getting your feet wet, I recommend starting here... http://www.amazon.com/gp/product/B00IC1FKO0?psc=1&redirect=true&ref_=oh_aui_d_detailpage_o05_
>The Integrity of the New Testament is a presentation of evidence for objective students, affirming that it is what it claims to be. This book is part of the Expository Files Expedition series published by Expository Files. The writers have contributed chapters based on their areas of expertise. Chapters discuss textual criticism, canonicity, testimony from the church fathers, date and authorship, text variances, translation and related subjects, with an excursive exposition of Luke chapter 1:1-4.
The above book is pretty good, give it a try.