It's hard to imagine a book of Christian doctrine as important as this one, at least among those written in the last 450 years. John Calvin's magnificent tome is essentially the canonization of Reformed Protestant theology, and the primary handbook of beliefs for Presbyterian and Reformed churches around the world.
Yes, The Institutes of the Christian Religion is very long. How could it be otherwise? Calvin's project was to solidify and express the essential doctrines of the pure Christian faith, and he managed admirably. The Institutes is poetic, sound, and highly devotional; here there are no dry or divisive academic rants, just theology as it was meant to be done—meant for the improvement of its readers, and eminently biblical.
Originally published in 1536 (in Latin), Calvin's work was the definitive codification of Protestant doctrine, and remains so to this day. Unlike many Catholic books of a similar nature, this one relied neither on secular philosophy or human tradition (sacred or otherwise); instead, Calvin's referent was the Holy Scripture, and to that alone he paid allegiance.
Church tradition is not completely ignored, however. The Apostles' Creed has long been identified as the briefest encapsulation of orthodox Christian beliefs (containing everything a person must believe to be considered a Christian), and Calvin organized his material according to the flow of the ancient Creed. This demonstrates his own orthodoxy, as well as making it fairly easy for those familiar with the Creed to follow his arguments and structure.
Part one deals with God the Father; part two concerns the Son; part three is about God the Holy Spirit; and the fourth and last section illuminates the doctrine of the Church, Christ's body. This structure underscores the secondary nature of anthropology (the doctrine of man) to theology (the doctrine of God), as well as setting a deeply Trinitarian precedent for churches in the Reformed tradition.
Christians of any denominational background will find little to disagree with here, though Calvin's Reformed articulation of the faith is losing ground to more "Evangelical" strains. However, all Christians would do well to read The Institutes of the Christian Religion, for its historical value, for its doctrinal precision, and most of all for its commitment to encouraging Christian love, unity, and service to the God of all creation.