Nationalism is a good introduction to Gellner's theory of nationalism. To be clear, this is not a general introduction to nationalism. Apart from discussing his widely acclaimed modernist interpretation of nationalism, Gellner also engages with some arguments against his theory. In so doing, he locates and clarifies his intellectual position on nationalism. At the beginning of the book, he prepares the ground for his theory by discussing history of mankind as well as the role of culture and social organization in human societies over time.
Nationalism, according to Gellner, is mainly a product of industrialization. It is the homogenizing imperative of industrial economy that facilitates the emergence of nationalism in a society. He stresses the necessities of industrial society, such as general education for the development of manpower, the necessity of semantic/uniform communication for efficiency, creation of high culture as distinct from folk culture and so forth, as conducive to the formation of nationalism. He is careful not to deny the genuine passion that nationalist sentiments arouse. However, nationalist sentiments are not evenly shared in a plural society and the boundaries/symbols/narratives of nationalism are subject to contestation.
His chapters on Marxism, Islam and Nationalism are particularly interesting. His discusses the decline of Marxism and the ascendance of Islam in the modern world, where secularization was supposed to have prevailed. For Gellner, Islam is marked by "a high level of modernity" (monotheism, low load of magic, non-mediation, transcendentalism) which make it adaptable to the modern world unlike Marxism which is characterized by "total modernity" (elimination of sacred/profane bifurcation, sacralization of this mundane world) which does not satisfy the prevalent social conditions. Islam, in this view, can be a competitor to nationalism but nationalism may also work hand-in-glove with any religion. In the latter case, religion is often invoked in nationalist assertions as as form of identity rather than as a matter of dogma.
Gellner opposes any enlightened preaching against nationalism. He argues that some places and people simply need nationalism more than others. In the last chapter, he teases out some practical implications of the understanding of nationalism. Before effecting any radical change inspired by nationalism, he underlines that nationalist movements or any government should be guided by considerations such as stability/continuity, affluence/economic growth, de-fetishization of land, acceptance of pluralism, balance between centralization and decentralization, and so forth.