In this book Foucault wished to both show his own way of looking at things and to distance himself from his previous more structuralist leaning book on the same subject, THE ORDER OF THINGS. The book is more of an interesting insight into how Foucault did research for then it is a way of showing how someone else can go about imitating his style of post-structural archeology.
Foucault tries to outline throughout the book both how and the importance distancing histories from subjects and eras. He would rather and did focus on the discourse itself, but not just privileged discourses on the surface, the deep underlying genealogies that stem from rules, processes and agreed upon or forced barriers that underlay a topic. Foucault of course throughout his career used his perceived method to trace how power institutions came to use knowledge from the enlightenment to present as an ever encompassing tool of political and theory laden subjective strands that now masquerade themselves as the methods of institutions in a neutral/objective fashion.
In a sense disguising their power in the rhetoric of objectivity and science whether the institutions methods really warrant that or not given their previous genealogical history. I say genealogical because Foucault by admission borrows heavily from Nietzsche in instituting his perceived method of archeology and in his focus on power relations through his short histories.
This way of doing things is not only in opposition to his own earlier writings, he is also implicitly going against the likes of Marx, Sartre and Hegel. He does so in outlining the impossibility of an overarching meta-historical position. The size and interconnectedness of discourses is just too large and amorphous to really pin down a teleological element or epochal formation that makes any sense. Also a general criticism against vanilla structuralism and his own earlier idea of "epistemes."
He criticizes the teleological wish that every subject in the humanities, become a "true science" in some sense. Saying that they may not be unfinished subjects, but rather alternatives to science as a way of describing the world.
In the appendix they added his seminar DISCOURSE ON LANGUAGE, in my edition of the book. It seems to be a helpful more easy to understand shorter version of what he said throughout the book before. Playing a similar role to STRUCTURE, SIGN AND PLAY, in Derrida's writings about his own deconstructive way of reading. I thought the questioning of Foucault's, can we have a philosophy in some way which is not Hegelian in this essay a strange, but thought provoking one. If what Foucault means is, "can we now have a philosophy which is ahistorical after Hegel?" I'm not exactly sure, seeing as anti-foundationalist stances are always in some way based on humanity's relation to it's own knowledge, Foucault may have a point in that all philosophy after this point will have to take historical discourse into account, since foundationalism is no longer an option.
The thing is I agree with a lot of what Foucault says, but I'm not sure anyone else could use this method other than Foucault. There aren't any obvious flaws in what he says or how he lays things out, yet I'm sure even the most diehard foucauldian, would have trouble really implementing a history under the same parameters that Foucault himself did in any kind of exactness. Still a worth while read for those interested in how Foucault viewed himself and his own work. I would recommend to those with an intermediate level of knowledge in philosophy and also after having read some of his histories. As what he says makes a lot more sense if you understand the examples he lays out from his other works.