I've started compiling the below list of the tools I think should be in every publisher, writer, editor, and author toolbox. I'm assuming you already have a laptop, the Internet, and an inspiring place to work. These should all be next on your list.
Garner's Modern American Usage
This is one of the top five most important books I own and would definitely make the desert island cut. It is both a desk reference and an entertaining read for anyone who self-describes as a book nerd, English major, or writer.
Papermate Flair Felt Tip Pens
Just trust me. These are the best pens for editing, proofing, marking up, and writing. I know 95% of our work is done digitally these days, but for the other 5%... Go with these badboys.
Stephen King's On Writing
I have only read two other Stephen King books. So, I'm not fanboy. However, this is the best writing memoir I've ever read. It is fascinating, helpful, hopeful, and realistic. Buy it, read it, give it.
Chicago Manual of Style
This is the manual for how to make sure consistency is kept across the board on all projects. Serial comma? Em dash? Semi-colon?
There are other options, but this one covers all the bases. It is also updated with regularity.
Get it in print or digitally. Just get it.
Huge World Map
In a world where publishing has now very much become a global force, you've got to keep the world in front of you.
If you don't keep your readers at the center of what you do, you'll lose sight of them.