I have 3 different copies of Machinery's handbook and use them almost every day.
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 few reasons:
- can print/reproduce any pages in whatever paper size you want (great for looking at the smallish figures/graphs)
- can search and find anything almost instantly
- no more lugging around (or losing) those rather-bulky tomes
- can share (over a network)
- easy to copy-&-paste