If you want to write fiction, then there is one thing I would recommend you have above all others, one thing that is truly a magic wand that will improve our writing more then anything else: a good grammar guide.
A good grammar guide is really indispensable. If you can manage to read and learn most of the basic rules, you’ll be head-and-shoulders above everybody else. Agents and editors might say that they don’t care about a typo here or there, but that’s really all they mean. A typo or two and few grammatical errors; that’s about it. If your writing is riddled with grammatical errors, it becomes basically unreadable to editors and agents, and they immediately stop reading it, unless you’re an extremely famous celebrity!
So here are some basic style and grammar guides, the best of the best. I highly recommend that you get one and familiarize yourself with the rules. There really isn’t any excuse not too!
The Elements of Style by William Strunk (Author), E. B. White (Author), Roger Angell
The Chicago Manual of Style: The Essential Guide for Writers, Editors, and Publishers Chicago Editorial Staff
The Gregg Reference Manual: A Manual of Style, Grammar, Usage, and Formatting Tribute Edition by William A. Sabin
The St. Martin’s Handbook by Andrea A. Lunsford
Amy M. Newman 
I like the Harbrace Handbook too. 🙂