Joseph F. Zimmerman, Horizontal Federalism. Interstate Relations, State University of New York Press, 2011

The book I am going to review is the brand new volume by Joseph. F. Zimmerman, Professor of Political Science at Albany State University of New York and an outstanding scholar in the field of federal studies.
This book represents just the last link of a longer scientific chain that has led Zimmerman to investigate the burning issue of interstate relations in the US over the years.
Despite its general title it is not a volume on comparative federalism or comparative constitutional law as it looks only at the US experience.
This volume has a twofold goal: on the one hand, the Author aims to give an exhaustive overview of the phenomenon of interstate relations in the American context by providing “detailed information on interstate relations and to analyze their various facets” (p. x); on the other hand, the book has also a normative dimension, since it tries “to advance recommendations to improve the economic union and the political union” (p. x).
The result is a brilliant book which offers a clear but at the same time problematic (since it stimulates questions and new research in this field) view on the state of American federalism.
The starting point of his work is the idea of “contact” between levels that represents a constant element in the progressive shift from a pure dualist view of federalism to the more complex cooperative federal arrangement, as the incipit of the first chapter says “The division of political power in a federal system, between the national government and the states, automatically produces relations between the latter. These horizontal relations may be cooperative as manifested by interstate compacts, uniform state laws, reciprocity statutes, administrative agreements, and regional and national associations of state government officers. Such relations, however, can be hostile” (p. 1).
Cooperation and competition are thus at the heart of the American federal arrangement and starting from this assumption the Author is going to engage with the analysis of some relevant areas where it is possible to find confirmation of this.
After having recalled the “interstate constitutional principles” (p. 10 et seq.) in the first chapter, in Chapter 2 Zimmerman moves to the analysis of the role of the US Supreme Court, acting as a  referee in the interstate controversies.
As the Author says at the end of the second chapter, interstate disputes may be solved through an alternative dispute mechanism or thanks to other arenas.
Indeed, in Chapter 3 Zimmerman offers a very interesting account of the interstate and federal-state compacts, presenting a detailed typology of them (p. 45 and seq.).
Chapter 4 is devoted to the full faith and credit clause, a fundamental US provision analysed from the perspective of interstate cooperation.
Chapter 5 focuses on the privileges and immunities clause by taking into account states’ attempts to deny privileges and immunities to residents of other states.
This chapter illustrates the delicate equilibrium between the necessity to guarantee interstate comity and to overcome barriers and discriminations in terms of fundamental rights of the individuals.
The rendition clause is at the heart of Chapter 6, whereby Zimmerman engages with the relevant case law of the US Supreme Court and the role of the legislatures and interstate agreements in this field.
As stressed by Bowman (Ann O’M. Bowman, Horizontal Federalism: Exploring Interstate Interactions, Journal of Public Administration Research and Theory, Vol. 14, no. 4, 2004, pp. 535-546) “theories of horizontal federalism take interstate competition as their starting point” and indeed the idea of interstate competition is key in the analysis of the “Mercantilism problem” (p. 117) and the role of the American judiciary in the progressive removal of trade barriers among states is treated in Chapter 7. However, as Zimmerman writes on page 138, protectionism does not exhaust all the existing forms of interstate economic competition; that is why the Author devotes another chapter (Chapter 8) to the issue of interstate competition for attracting business firms, tourists and gamblers.
Chapter 9 is on interstate tax revenue competition and deals with the “tax differential problem” (p. 159 et seq.) and other related issues. This is another very sensitive field: the US Constitution, on the one hand, seems to favour tax regime variety but on the other hand this phenomenon may create interstate frictions, jeopardizing the federal compact.
In Chapter 10 Zimmerman goes back to the possibility offered by interstate cooperation, by looking at the role played by the National Conference of Commissioners on Uniform State Laws, by ad hoc informal interstate administrative agreements, other national associations or even by the Congress in the promotion of uniform state laws in some sensitive areas.
Finally, Chapter 11 offers a list of concrete proposals for improving horizontal relations, by giving a fundamental contribution to overcoming the existing gap between the importance that this area has de facto and the poor visibility it has on the political agenda of American federalism.
This book is definitely recommended for those interested in American federalism but also offers many interesting points for comparative lawyers although it is limited to the US case. The first and the last chapter, for instance, give an interesting perspective on the relation between competition and cooperation or negotiation and conflicts in federal arrangements and even the solutions proposed in the last part of the book might be of interest to scholars willing to contribute to the development of intergovernmental relations.