This is a free chapter, from an unpublished 
textbook by Shelia Kennison of Oklahoma State University, that you can either read online or download as a pdf document.
The chapter covers a range of ideas and issues focused on the research process:
• different research methodologies
• causality
• experimentation
• representative sampling
• reliability and validity
• Type I and Type II errors
• ethics
The text also includes a couple of pages of “key terms” plus a set of questions based on the text designed to assess student understanding.
While it’s not exactly ground-breaking in terms of content and design it seems solid enough for A-level / AP Psychology.