From the reviews:
"Volumes III and IV complete L. Hörmander's treatise on linear partial
differential equations. They constitute the most complete and up-to-date
account of this subject, by the author who has dominated it and made the
most significant contributions in the last decades.....It is a superb
book, which must be present in every mathematical library, and an
indispensable tool for all - young and old - interested in the theory of
partial differential operators." L. Boutet de Monvel in Bulletin of the
American Mathematical Society, 1987
"This treatise is outstanding in every respect and must be counted among
the great books in mathematics. It is certainly no easy reading (...)
but a careful study is extremely rewarding for its wealth of ideas and
techniques and the beauty of presentation." J. Brüning in Zentralblatt
MATH, 1987
Honours awarded to Lars Hörmander: Fields Medal 1962, Speaker at
International Congress 1970, Wolf Prize 1988, AMS Steele Prize 2006