One of the best purchases a freelance writer can make is a copy of the AP Stylebook (and maybe copies of a few other style guides). It comes down to the fact that, while most magazines and newspapers have their own little quirks, in general AP style is used to keep writing consistent.
You don’t have to run out and buy the most recent copy, either. AP style has stayed mostly consistent over the past decade. If you do get an older copy, some of the changes have been posted online. You can even find most of the basics of AP style online, although there are some style issues that I would have considered basic but were only in the book.
Knowledge of various styles can be a selling point on your resume or query letter. It’s a fairly minor investment, as well — Amazon lists used copies for $2.45.