This is just one amateur's opinion...
Any edition of "Machinery's Handbook" will be a great addition to your library. The "Machinery's Handbook Guide" is a great supplement to the handbook. The books cover a massive range of topics including: mathematics, mechanics, strength of materials, machining operations, and manufacturing processes. These can be bought online, but shop around for a better price.
You don't need the absolute latest edition, so don't pass up an earlier revision if it's really cheap. I'm pretty sure the latest revision is 26, as I haven't seen anything newer.