"The Psychology of Man's Possible Evolution" by Peter Ouspensky

This is a short book - no more than a 100 pages in most editions.  It contains five lectures given by Ouspensky to his pupils in England.  There is little to be said about the book itself, except that it expouses similar concepts to those found in Fragments of Unknown Teaching, but in a more general and explanatory way.  In the same vein though, the explanations in this book are much more theoretical, on purpose, than the material in Fragments Of Unknown Teaching.

The First Lecture sets the stage for exposition of latter, more complicated ideas of Gurdjieff.  The Second Lecture is probably the most valuable, although not the longest, and talks about the different types of consciusness, the different types of men, the four types of goals for a seeker of truth and the four harmful manifestations in every man-machine that need to be observed and overcome in order for the human evolution to be able to move to the next level.

The third and fourth lectures further elaborate on some of the main thesis presented in Gurdjieff teachings, and makes frequent references to the Fragments of Unknown Teaching (which was published after Ouspensky's death, while this book was published while he was still alive).  The fifth lecture was written in a later period than the previous four, and concerns itself chiefly with how to recognize proper schools of the Fourth Way.

The main tenet that Ouspensky makes painfully and tenaciously clear throughout the entire book is that no person can make any progress without being involved with a proper School of the system.