The text of this book is based on transcribed lectures given by Walter Baade in 1958. The notes were edited and put into final form by Cecilia Payne-Gaposchkin.
The book is interesting for its description of the difficulties Baade had establishing the magnitudes of variable stars in M31 -- these difficulties led him to realize that Cepheid variables and cluster variables were formed from two different stellar populations that had different chemical histories. This division into population I and population II was a crucial step in understanding stellar evolution and the formation of galaxies.
The book was from lectures given to other astronomers so the reader should know something about astronomy before trying to read it. The character of Baade comes through though and he was a amazing observational astronomer with his own opinions. The book is a snapshot of a time when our understanding of the galaxy was changing and is from one of the men who made it change. It is well worth reading.