Well, buying a book will not transform you into Alex Webster, but the pieces of Information contained in this book, are definitely of use to people like me, who are in (/or are interested in joining) a band in the extreme metal genre.
His approach is always very "hands on", and he does not explain very much about the theoretic background (there are more then enough books that cover this part), a fact that I appreciate a lot.
Topics covered are:
-Technique:
Important Chords and scales widely used in extreme Metal (with examples), Right hand techniques (3 finger), left hand technique (excercises to get you hand in shape), Tapping (just check those insane Blotted Science Licks, here the technique is explained)
-Application:
Rhythm Excercise, Triplet Grooves, Sixteenth Notes grooves (3 finger technique), Writing Bass lines, Doom/Sludge, Song examples.
AND there's a CD that comes with this book, with some awesome backing tracks to play to. (midi files are delivered as well, to change the tempo of the backing)