A Writer's Reference:
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.