The Macmillan handbook of English

I've been a linguist/translator for over 20 years (English is my native language). I've used literally hundreds of reference books in my life, and for the average user (H.S. and college students, parents helping kids with homework, people proofreading their own or colleaques' work...) I would rank this as one of the two best English grammar books I have ever encountered. It is well organized, so it is easy to find the right section. It is clearly written, so you don't have to decipher the text. It indicates whether particular items are required by grammar rules, or merely prefered, so you don't have to wonder if something could be phrased differently if you want to. I give it my highest recommendation!