I think Diabology is still the best all-around backgrounder on modern diaboloing. Some of the highly technical stuff like the 3d, 4d, and integral stuff has progressed a fair bit since then, but what they present is still a solid foundation and then some. You will get bits and pieces from online videos and if you have other diaboloists around at clubs or festivals you may pick up what you're missing there, but otherwise (and even in spite of this) the DVDs could be useful. They're still well-done freestyle videos too.