"Official and Unpublished Correspondence Concerning the Determination of Boundaries Between Paraguay and Brazil" is a valuable collection of letters that reflects important historical events and diplomatic relations between the two countries. The author of the work is the outstanding researcher and diplomat Felix de Azar. The book recreates the chronology of events, discussions and agreements that influenced the definition of the borders between the two countries. This publication is an important source for understanding the history and politics of the region, and also has great scientific and cultural significance.