amazon.com
http://www.amazon.com/Yellow-Green-B.../dp/B009BA1UJK
amazon.ca
http://www.amazon.ca/Yellow-Green-Be.../dp/B009BA1UJK
It looks interesting. As for why this is free, perhaps the author thinks it is the best introduction. The first sentence in the Introduction:
"This is my favorite volume of all three books, by far."