I recently stumbled across the CopyHackers site, which is focussed on helping people to write better quality copy for websites and other promotional materials. A lot of their recommendations are really about the mindset of the copywriter, and most of the content is relevant to any form of communications with customers or investors.
They have some great free cheat sheets and I’m enjoying their e-mail newsletter which is filled with tips (sign up at the top of their page).
If you ever write anything to communicate your idea or product to others, take a look!
(I have no affiliation with these guys – I just like their content and their business model seems like an interesting new way of selling copywriting expertise!)
