Alexander Nagel & Elias M. Stein 
Lectures on Pseudo-Differential Operators [PDF ebook] 
Regularity Theorems and Applications to Non-Elliptic Problems

The theory of pseudo-differential operators (which originated as singular integral operators) was largely influenced by its application to function theory in one complex variable and regularity properties of solutions of elliptic partial differential equations. Given here is an exposition of some new classes of pseudo-differential operators relevant to several complex variables and certain non-elliptic problems.
Originally published in 1979.
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