"The appendix is very valuable and helps the reader to find an orientation in the very voluminous literature devoted to the spectral theory of differential operators ... anybody interested in the spectral theory of differential operators will find interesting information in the book, including formulation of open problems for possible investigation." Mathematical Reviews "This book is well-written, and a list of symbols and the index prove useful. A substantial number of open questions is also included. Although addressed primarily to the research community, the book could also be used as a graduate textbooks." Zentralblatt MATH