This monograph deals with products of Dedekind's eta function, with
Hecke theta series on quadratic number fields, and with Eisenstein
series. The author brings to the public the large number of identities
that have been discovered over the past 20 years, the majority of which
have not been published elsewhere. The book will be of interest to
graduate students and scholars in the field of number theory and, in
particular, modular forms. It is not an introductory text in this field.
Nevertheless, some theoretical background material is presented that is
important for understanding the examples in Part II of the book. In Part
I relevant definitions and essential theorems -- such as a complete
proof of the structure theorems for coprime residue class groups in
quadratic number fields that are not easily accessible in the literature
-- are provided. Another example is a thorough description of an
algorithm for listing all eta products of given weight and level,
together with proofs of some results on the bijection between these eta
products and lattice simplices.