I am currently maintaining a random (but useful) archive of (mostly mechanical) engineering references using http://docs.google.com" , which is an amazingly useful (and FREE) service.
Anybody can access it for free by going here...
Can you please explain why you'd need more than one? The only reason I could imagine is if you have them in different locations (e.g. desk, plant floor, etc...).
I recently got the a digital (CD) copy of the 28th Edition, and I find it to be far-superior to the old-school paper book for a...