I borrowed this from my father and definitely need a copy for myself to markup and keep on my main bookshelf.
Clean, Well-Lighted Sentences: A Guide to Avoiding the Most Common Errors in Grammar and Punctuation by Janis Bell is an excellent handbook for writing, and has easy-to-understand explanations, samples and quizzes to illustrate and answer questions about common errors in English.
I found the following particularly helpful:
who v.s. whom
why there is an "ing" after "helping" in "we appreciate your helping us with..."
explaining gerunds
"I wish I was" or "I wish I were"?
common mass nouns (no s on the end, e.g., feedback, advice, homework).
I put a period after that last item on the list to mark the end of the sentence, as shown in an example in the book :)
The explanations are not highly technical, making the book approachable and enjoyable to read. Highly recommended!
