A quality source that I found to be very 'user friendly' covering chord/scale relationships, application of theory to improvisation, etc. is Marc Sabatella's "A Whole Approach to Jazz Improvisation and the website is (http://www.outsideshore.com/primer/primer/index.html). Another source that has helped me alot are publications by Jamey Aebershold (www.jazzbooks.com). Hope you find these helpful...I know these gave me a much better perspective on theory and modal improvisation and in language I could understand. ...Lefty