I have recommended this book before on this board, but I will do so again: Being-in-the-world: A Commentary on Heidegger's Being and Time Division 1 by Hubert Dreyfus. It makes some sense of what Heidegger is up to in Being and Time, and it is rather readable. If you have even a little interest in Heidegger, read this book.
Dreyfus goes through Division 1 slowly and he renders intelligible much of what seems really obscure in those sections of the book. Dreyfus only briefly touches upon the "existentialist" sections of Being and Time in Division 2. Some may think these parts of Being and Time are really important and perhaps also believe that Division 1 cannot be interpreted as independent of Division 2 in the way Dreyfus does. However, essentially all of the central content of Division 1 had been covered in lecture courses prior to the publication of the book, but we see none of analyses of anxiety, death etc. that we find in Division 2. These themes come up only in a lecture course delivered a couple years after the publication of the book where they are treated rather differently.
Heidegger was gathering recognition as an important philosopher because of his lecture courses at Marburg where he was treating themes that found their place in Division 1, but he did not feel content to publish anything. But he was getting close to a place where he had to either "publish or perish". In order to get tenure he had to publish a book and Being and Time is the result of Heidegger's effort to quickly get tenure. He couldn't publish just Division 1 because he was asked to produce a longer work. He therefore had to think of the content of Division 2 rather quickly and he composed it much more sloppily than he did Division 1 and it shows.
It is very easy to find more to relate to Division 2 than Division 1 as it deals (among other things) with phenomena that appear highly significant to us, like anxiety, guilt etc. But there is little cohesion throughout and connections between different ideas he presents are usually tenuous. Also what he means to be talking about when he discusses time is almost completely opaque.
Division 1 is not much easier to read than Division 2, but it is better worked out and more cohesive. Heidegger brought forward many new ideas there and it pays to consider them carefully. Heidegger took himself to uncover certain fundamentally mistaken assumptions in the philosophical tradition from Plato all the way down to Husserl. Dreyfus collects these under 5 headings in his introduction and says that the fact that Heidegger made these assumptions explicit is reason enough to devote time to read his work.
1. Explicitness. "Western philosophers from Socrates to Kant to Jurgen Habermas have assumed that we know and can act by applying principles and have concluded that we should get clear about these presuppositions so that we can gain enlightened control of our lives. Heidegger questions both the possibility and desirability of making our everyday understanding explicit. He introduces the idea that the shared everyday skills, discriminations and practices into which we are socialized provide the conditions necessary for people to pick out objects, to understand themselves as subjects, and generally to make sense of the world and our lives."
2. Mental Representation. "Heidegger questions the view that experience is always and most basically a relation between a self-contained subject with mental content (the inner) and an independent object (the outer)." For him "there is a more fundamental way of being-in-the-world that cannot be understood in subject/object terms. Heidegger does not deny that we sometimes experience ourselves as conscious subjects relating to objects by way of intentional states such as desires, beliefs, perceptions, intentions, etc., but he thinks of this as derivative and intermittent condition that presupposes a more fundamental way of being-in-the-world that cannot be understood in subject/object terms."
3. Theoretical Holism. The underlying conception of the mind in the philosophical tradition led "to the notion of a holistic network of intentional states [i.e. mental states which are about something], a tacit belif system, that is supposed to underlie every aspect of orderly human activity, even everyday background practices. [...] Heidegger opposes this philosophical move. He denies the assumption that there must be a theory of every orderly domain - specifically that there can be a theory of the commonsense world."
4. Detachment and Objectivity. "From the Greeks we inherit not only our assumption that we can obtain theoretical knowledge of every domain, even human activities, but also our assumption that the detached theoretical viewpoint is superior to the involved practical viewpoint. [...] The pragmatists questioned this view and Heidegger can be viewed as radicalizing the insights of such pragmatists as Nietzsche, Pierce, James, and Dewey."
5. Methodological Individualism. "Heidegger follows Dilthey in emphasizing that the meaning and organization of a culture must be taken as the basic given in the social sciences and philosophy and cannot be traced to the activity of individual subjects. Thus Heidegger rejects the methodological individualism that extends from Descartes to Husserl to existentialists such as pre-Marxist Sartre and many contemporary American social philosophers."
Of course what these assumptions really come to is explained in depth in Dreyfus' book. Even though I find some of his attempt to spell these assumptions out less than fully satisfactory (especially regarding the second assumption about mental representation above), I think he almost always starts on the right foot in approaching these issues.
In his earlier book on artificial intelligence, What Computers Can't Do? Dreyfus did a close study of the then-ongoing research for creating intelligent machines and argued that that research was based on assumptions similar to those outlined above. Then he used arguments similar to those Heidegger provides in Division 1 of Being and Time to show that that line of research could not lead to the development of intelligent machines. His criticisms have actually turned out mostly correct in their essentials. That research program is dead at the moment. Other lines of research are possible for artificial intelligence, but these have had to learn the lessons from the failure of the classical research program. Some people in the area of artificial intelligence have therefore read some Heidegger - at least through Dreyfus' interpretation. Everyone in our artificial intelligence lab in college had to know this stuff even if they weren't too into the more theoretical part of our work.
