The stuff at the site that mario posted gives some simple basic info, but it you don't get the nitty gritty-- Alan Lomax's wonderful book, "The Land Where the Blues Began" is the definitive discussion--it is filled with interviews, personal insights(he collected, recorded, and played) and very useful desriptions of the musical elements--this book is such an important document that it will be referenced and addressed in every serious academic discussion of the blues--