Come on, admit it, you either still double-space after periods when you type, or you know someone who does...
The PC is Not a Typewriter (and its Mac companion*) is a fast, fun, easy read that will help you make your written communication more professional and sophisticated. Yes, it references older technology, but the technical info isn't the important part here - it's the simple, clear information and inspiration for crafting of better writing. It covers punctuation, leading, special characters, kerning, fonts, justification, and so much more.
Read this, keep it nearby, refer to it often - your writing, and your readers, will thank you for it.
Frankly, these 96 page-long gems are the books I would give to everyone I meet, if I could.
*Why is there a PC and a MAC version? My guess is 1) for the technical references which I'm sure were helpful when it was released 2) to get the full audience - some PC people won't pick up something targeted at the MAC audience, and vise versa.