Following up on Kirk's good advice, I know a couple of speed freaks who practice arpeggios rather than scales. (Well they do both, but they say arpeggios are more interesting for solo work.) So learning chords is essential for that, and Kirk's book might indeed help.
Also check out Troy Stetina's "Speed Mechanics for Lead Guitar" book. He's got some really good books for rock and metal styles. I use his "Total Rock" book, which is more of a general rock orientation rather than metal/shred, and I think it's very good. Just stay clear of his DVDs - poorly designed in my opinion.
http://www.stetina.com/lessons.html