These aren't "preachy" resources, nor or they the most dull, drab, boring ones available. Two of them even have a sense of humor.
The reason I list these is that, looking at some of the Warriors for Hire threads as well as the "help me" writer threads on the main forum, it looks like there's a desperate need for quality writing books.
These resources are perfect for IM'ers and freelance writers time-wise as well. In other words, it won't take you all year to get through them.
So without further ado, here's my three favorite resources. And there's no affiliate links - just product links.
1. The Associated Press Guide to News Writing
Amazon.com: The Associated Press Guide to News...
Forget that you're not a journalist. Being a journalist or a freelance writer or content creator has nothing to do with the way you write - inherently. Of course, there are many intrinsic differences, but developing an easy to follow writing style is something that is valuable in any niche.
This resource provides plenty of humor mixed with a number of ways to make your writing more colorful without being overly sarcastic. Trust me, your readers will thank you.
2. Jensen's Punctuation
Jensen's Punctuation: Frode Jensen: 1886061262: Christianbook.com
No grammar frills. This book just teaches you how to structure sentences using the proper punctuation marks. Like I mentioned in another post, the information I learned in this book has stuck with me for years. It's seriously the only real punctuation help I've ever gotten.
3. The Elements of Style
Introductory. Strunk, William, Jr. 1918. Elements of Style
The really good edition is mostly E.B. White's work, but the original is fine as well. It's available for free here.
Edit: For those who are looking for an actual grammar program, I also recommend Jensen's Grammar. You can check out the specifics here: