Helmut Koester: Introduction to the New Testament, Volume One: History, Culture and Religion of the Hellenistic Age (Fortress Press/Walter de Gruyter, 1982).
Helmut Koester: Introduction to the New Testament, Volume Two: History and Literature of Early Christianity, Second Edition (Walter de Gruyter, 2000).
Heikki Räisänen: The Rise of Christian Beliefs: The Thought World of Early Christians (Fortress Press, 2010).
Hans-Josef Klauck: Apocryphal Gospels: An Introduction (T&T Clark, 2003).
E.P. Sanders: Judaism: Practice & Belief 63 BCE—66 CE (SCM Press, 1994).
Religions of Late Antiquity in Practice, edited by Richard Valantasis (Princeton University Press, 2000).
Everett Ferguson: Baptism in the Early Church: History, Theology, and Liturgy in the First Five Centuries (Eerdmans, 2009).
For reasons too mysterious for a graduate student to comprehend, I struck a deal with my faculty advisor: study credit points (necessary for me to someday complete that PhD) in exchange for writing a public reading diary of sorts in this very blog.
The effort has two main goals. First, by reading through comprehensive introductions from start to finish to the field of early Christian studies draws my attention to any blanks I might still have in my growing expertise of the area. Second, by combing through works of a number of top scholars in the field will (hopefully) teach me to further appreciate the subtle differences in their respective approaches, even though all of the authors above are representative of the same general school of thought (more on that below).
Besides, the summer's quite fine out there, and one can easily spend the hours under a tree, and to withdraw to the library with air conditioning only when (if) the heat outside becomes unbearable. In Finland, that's about as soon as 25 degrees of Celsius gets broken.
Since Koester's introduction is the most comprehensive in scope, I will begin with that, while everything that follows will be compared to it.
Biblical studies is taught worldwide in a variety of contexts, from denominational seminaries to secular universities. It is hardly the same to come to one's initial understanding of the issues in the study of early Christianity from reading the books of Ben Witherington III, or N.T. Wright, or Gerd Lüdemann. The curriculum in the University of Helsinki follows one particular trajectory, and has prominent places for the works of Walter Bauer, Helmut Koester, E.P. Sanders, Gerd Theissen, Hans-Josef Klauck, and—naturally—Heikki Räisänen, the grand old man of Finnish exegesis. Räisänen, in fact, more than anyone else, could be considered as having a school of thought of his own in the University of Helsinki. At least my perception picks up his influence in everyone in the department.
The particulars of this school are roughly the following. Jesus was a failed Jewish prophet of the imminent apocalypse. Paul was a somewhat more successful Jewish preacher of the imminent apocalypse, though even he had some trouble in convincing some of his fellow believers of his superior understanding and interpretation of the Jewish scriptures. (Though outside the historical study proper, it could still be noted that contemporary presentations of Jesus and Paul in various Christian denominations have little in common with their first-century counterparts.) From early on, there existed a number of different Christianities, some of which were quite dissimilar in a number of important details. And above all, it is not a historian's solution to dismiss the writings of those Christianities which remained non-canonized, following the centuries-long struggle for power and eventual triumph of one of those variants—the one we have been trained to think of as the Christianity, but which, judging from the ancient sources, has no more justification for that title as any of the others.
From these considerations it follows that my reading of Koester, or any of those books listed above, is not going to rock my world and shift my perspective in anything but the most subtle of manners.
Like a summer blockbuster movie, come to think of it.