I just found a great resource for anyone who wants a brief introduction to text criticism of the Bible. Apparently, Our Bible and the Ancient Manuscripts by Sir Frederic Kenyon used to be a common textbook for textual criticism. I found it to be a very helpful summary of the important points. It can help you make sense of the text-critical apparatus in the Nestle-Aland Greek New Testament or Rahlfs’ Septuagint, even the Hebrew Bible–but the book was published in 1939, seven years before the discovery of the caves at Qumran. Hence the reason it is no longer a standard.
Kenyon’s book has been superceded by a few other books: Bruce Metzger’s Textual Commentary on the New Testament (2005) and his Text of the New Testament (co-authored with Bart Ehrman, 2005), Kurt and Barbara Aland’s Text of the New Testament (1995)and Emmanuel Tov’s Textual Criticism of the Hebrew Bible (2001).