I'm developing a course that focuses on the transistion from arithmetic to algebraic thinking, particularly in grades 5-8. We will do this through focus on the common core. I'm also putting together a collection of suggested readings from the math education literature. I would be interested to hear your suggestions for suggested readings.
Early algebra research necessarily deals with the development of algebraic reasoning and questions like "what is algebra" and "what counts as algebraic thinking and reasoning?" And my own readings on early algebra have helped me to focus on what about algebraic thinking are students developing, apart from the manipulation of symbols. This is where, I think, some of the early algebra research may have more general interest, since some of the authors deal with what students are capable of that is connected to algebra well before they are doing what we are familiar with in an Algebra 1 course.
A book collecting some different research on Early Algebra is:
There is a review of it in JRME (Chazan & Edwards, 2010), and someone is sharing the review here as a PDF (in case you don't have access).
From the review:
Even though you're not talking about algebrafying early mathematics, and depending on what sort of things you want students to consider and even argue about, this book (along with other early algebra research) may provide perspectives that spark thoughtful discussions and reflections.
Although these two NCTM books cover K-12 (there are items specifically directed at middle school level) they have ideas related to how to develop algebraic thinking: The Ideas of Algebra, K12, 1988 Yearbook, A. Coxford and A. Shulte, editors, and Developing Mathematical Reasoning in Grades K-12, 1999 Yearbook, Lee Stiff and Francis Curcio, editors.