Mine would have to be Speed Mechanics For Lead Guitar by Troy Stetina. I've had the book for a few years, and yet I still am able to go back to it and improve my playing.

I think videos can teach way better.

Speed Kills by Michael angelo is awesome for getting your speed up. I had played guitar for 5 years before i even knew about shred and classical and death metal , then i saw his video and was like "holy shit this guys crazy." He can get your speed up really well. Everytime i watch it i learn some new shit.

The only guitar videos I've really watched have been "In the style of..." ones by Curt Mitchell, and I hate them with a passion. In fact, that's what scared me away from videos, two years ago.

Videos and books are great, but pretty much everything you need is available via internet. I've got John Petrucci's Rock Disipline, Marty Friedman's Melodic Control, and one of Malmsteens videos along with various clips from various artists.

I don't feel that they offer speed improvments or anything like that, but more of inspiration to actually watch someone actually play what you are trying to learn, any new techniques.

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Yeah Paul Gilberts video packs alot of information and quite a few techniques

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I have melodic control but i didnt find that it really taught me too much. Sure the guy is great and all but he has his own sense of style that cant really be copied, his exoticness is amazing. Marty Friedman is one of the best.