This is a book review of The Expanding Universe: A Primer on Relativistic Cosmology by William D. Heacox. Why another introductory cosmology book? It has more material on General Relativity than probably any other book at the same general level, filling an otherwise vacant niche. Have no fear, though; this is limited to what is necessary in order to understand the derivation of the Friedmann equations. While such material is available in more advanced texts, most introductory cosmology books present them almost as a deus ex machina. The book also emphasizes theory over observation. The fact that cosmology is now a data-driven science has led to observational aspects being emphasized in many recent books. Basic theory is of course available in older texts, but these often contain archaic notation, series expansions appropriate for small redshifts, and discussion of topics no longer important. While all of these are important in the history of the subject, the beginner is better served by a modern introduction, with just a brief nod to older traditions. In addition to the extensive background information on General Relativity, the book offers a detailed overview of homogeneous and isotropic cosmological models based on GR, so-called Friedmann-Lemaître-Robertson-Walker models. Important topics which are usually found only in (some) more advanced texts, such as the various types of cosmological horizons and the evolution of the cosmological parameters with time (as opposed to just measuring their current values), are given more than just a brief mention. Despite the emphasis on theory, potential application to observations is never far away; "theoretical observational cosmology" is a good description of the bulk of the book. Extensions to the standard model, such as MOND, modified field equations, and inhomogeneous models are at least mentioned. The book is clearly written and there are only a few typographical errors and so on. The author, an emeritus professor at the University of Hawaii, where he founded the undergraduate astronomy degree program, has written papers in many fields in astronomy, but none on cosmology. This is probably responsible for the book's well-balanced approach; experts sometimes don't see the forest for the trees and/or put too much emphasis on their own areas of expertise, which is fine for a monograph but not for a primer. This is an excellent introduction to theoretical cosmology which nevertheless takes recent observations into account. Its emphasis on GR puts it in a class by itself for books at this level; nevertheless, the non-GR parts of the book stand alone. It leaves out more general background (which is covered well elsewhere) in favour of discussing details which often aren't mentioned at all, resulting in a good and compact introduction which does more than just scratch the surface.