Hi Nelissa, there are a couple of books that you may want to consider. Two books come to my mind: Krippendorff's Content Analysis: An Introduction to Its Methodology and Kimberley A. Neuendorf's The Content Analysis Guidebook.
I agree with @Faimau: Klaus Krippendorff's Content Analysis: An Introduction to Its Methodology is one of the most widely quoted resources in the field; it seems to be in its third edition (2012).
Textbooks are important to study to generalize your findings. However, these textbooks should be used commonly by many schools so that you can generalize your findings, you need to make sure that they are used by many portions of a given communities or schools.