Difference sets are of central interest in finite geometry and design
theory. One of the main techniques to investigate abelian difference
sets is a discrete version of the classical Fourier transform (i.e.,
character theory) in connection with algebraic number theory. This
approach is described using only basic knowledge of algebra and
algebraic number theory. It contains not only most of our present
knowledge about abelian difference sets, but also gives applications of
character theory to projective planes with quasiregular collineation
groups. Therefore, the book is of interest both to geometers and
mathematicians working on difference sets. Moreover, the Fourier
transform is important in more applied branches of discrete mathematics
such as coding theory and shift register sequences.