"Discourse on the Method" (full name: Discourse on the Method of Rightly Conducting One's Reason and of Seeking Truth in the Sciences by French philosopher René Descartes may very well be the first book of modern philosophy and a theoretical basis for modern science. It is the predecessor of Descartes' seminal work "Meditations on the First Philosophy" which is considered as one of the most important books in the history of philosophy.
Descartes' "Discourse on the Method" is groundbreaking in tackling the problem of the manner in which we acquire knowledge and attribute truth to it. Descartes problematizes our relationship with reality by opposing a perceiving subject with a perceived object. This opposition is new to philosophy (at Descartes' times) that never thought of man and the theoretical validly of his judgments of the material world as a problem (philosophy after Descartes has lost its innocence, and not for the last time). This, for Descartes, warrants the consideration of how we know our knowledge of the world to be valid, and his answer is that the validly of our method of acquiring knowledge will ensure the validity of that knowledge. The most basic of the rules of that method is, according to Descartes', "never to accept anything for true which I did not clearly know to be such." (Discourse on the Method, Part 2). This, in essence, is what we call today scientific thought which was, to a large extent, born in Descartes' "Discourse on the Method".
"Discourse on the Method" is divided (like Meditations) into six parts. Here you can find short and essential summaries of each of the chapters.
More about Descartes:The Philosophy of Renè Descartes - overview
Meditations on the First Philosophy- essential summary
Meditations on the First Philosophy- summary by chapter
Descartes' proofs of God in Meditations