I believe that one of the first things one must do to truly understand the תשב"כ is to try and identify its unity. I will try and express this unity in terms of the "what" and the "why" of חומש. In other words, "what" is the actual content and "why" or what is its purpose? There is no "right" answer to these questions (not to say they are not subject to critique) - as one develops in his understanding so to his answers to these questions will develop and change. However, I think it is an exercise that is worthwhile. I would love to hear how other people answer these questions.
To express the "what" of the חומש I will attempt to give the briefest possible synopsis of the entire book:
How the שבועה to אברהם אבינו was fulfilled through the שליחות/נבואה of משה רבינו ע"ה thereby enabling humanity to know ה'.
Explanation: I believe saying "enabling humanity" underscores the תורה's concern with humanity (not just ב"י) and also is one explanation for the inclusion of the first eleven chapters of בראשית.
The purpose of the חומש or the "why":
Enabling humanity to choose טוב.
Explanation: it is through the נבואה of משה רבינו ע"ה that humanity (first ב"י) can choose טוב. This is more than just choosing the ultimate טוב it includes all of the means necessary to stay on the דרך to attain that טוב. In other words, we learn not just an abstract point about what טוב is but we are given 1) the specific means to attain it - in the מצוות and 2) paradigmatic stories that provide us with the best framework to grasp the need for and purpose of such a system.