Ketiv and Kri
For a good summary in English of different opinions on the origins of Ketiv and Qere, see the article The Origins of Ketiv-Qere Readings by Michael Graves. In order to read the Hebrew, most people will have to download additional fonts. For those who have access to Leshoneinu, the last few issues have had a multi-part article on the subject by Yosef Ofer. Also see this chapter by Emanuel Tov. This Hebrew Wikipedia article is also fairly good.