As a celebrated writer of no small renown, I’m often asked by those who would imitate my astonishing career trajectory what writing references I keep within easy reach of my Mac (apart, I presume, from the vast resources that reside in my head).
For a quick roundup of writing basics, it’s hard to beat Strunk and White’s The Elements of Style. Sure, it has its detractors (or, what we in the business call “stupid whining pedants”), but commandments like “be clear” and “omit needless words” are sorely needed these days.
If you’re looking for Elements, try to find a used copy of the second (1972) or third (1979) editions, both of which can be had for around $3. It’s the best investment a writer can make.