A Writer's Reference:
https://www.amazon.com/Writers-Reference-Diana-Hacker/dp/1457666766/ref=sr_1_1?ie=UTF8&qid=1470288113&sr=8-1&keywords=writer%27s+reference
Essential English Grammar:
https://www.amazon.com/Essential-English-Grammar-Language-Guides/dp/0486216497/ref=sr_1_1?ie=UTF8&qid=1470288171&sr=8-1&keywords=essential+english+grammar
Strunk is great, but I prefer the two above. Trying to "read" Strunk & White will bore you.
A Writer's Reference has an easy layout to quickly find the answer, and a great index. It has several sample letters too.
Essential English Grammar is a concise simple book. And it's cheap.
If you want to read interesting stuff, check out lettersofnote.com
Just interesting "rules". May be useful.
www.schoolsoutwashington.org/userfiles/file/b12writingasmeansofdiscovery.pdf