This is Michael Waski’s book, Teaching Algebra to the Adolescent: A Montessori Approach, Volume 2, revised and updated with new lessons and a new design! Volume 2 focuses on Later Algebra concepts.
 
Continuing the Montessori tradition of a discovery approach to mathematics through hands-on, student-driven work, this book continues that legacy with lessons specifically designed for the older adolescent (15-18y.o., or high school).  These topics and lessons are a continuation of all that has come previously, and in a way is a capstone to a student’s journey through mathematics.
 
While the topics are more advanced, this is not necessarily because of their complexity, but more because of the previous knowledge students generally need to have accumulated before doing this work.  Therefore, many of the lessons, especially the historical and foundational aspects, may be appropriate for younger students who have a passion and interest in mathematics.