This book is a classic. I think it's the book most often recommended for writers. What it has to say is good, but I suspect that part of the reason it is recommended so often is because it is so short!
The book teaches people to write in a plain, straightforward style. It consists of a series of rules and examples (that's the "Strunk" part) and an essay (that's the "White" part).
I think the most famous rule in the book is "Omit Needless Words."
There are points the book makes that you could argue about, but I think it's still essential reading for everyone who is interested in writing. And it's fairly painless to read -- or reread -- because it's so short. I've lost track of how many times I've read it, and I think it would do me good to read it again.
You can find copies of this selling very cheaply in almost any used bookstore or online. There is also an illustrated version that was published recently, which might be nice to get if you are giving this as a gift.