“The book is really well written. Each chapter and every section contains discursive paragraphs where the authors introduce the reader to the general content of the text and guide them through the path of the following theorems and proofs. ... For sure, this book contains a wealth of mathematical ideas which are worth being explored by working mathematicians.” (Marco Bramanti, Mathematical Reviews, June, 2025) “The complete set of volumes promises to deliver most of what is known about solving elliptic equations and systems on various kinds of flat domains under minimal conditions on the flatness (local and global) of the domains.” (Raymond Johnson, zbMATH 1523.35001, 2023)