Adding an s to a word may be enough to completely change that word. With other words, however, the change is not so big. When it comes to regard and regards, does that little s make a difference?
To S or Not to S
I'm writing to you in regard to a book review.
Regard is sort of an old-fashioned way to say about or concerning. Or is it regards? Use it in the sentence, and it still makes a lot of sense.
I'm writing to you in regards to a book review.
In the example, the correct usage of the word is regard. You're talking about a book review, a singular object. This means that any modifier of that noun should also be singular -- so leave off the s.
But in the plural, you want to add the s to regards. Let's look at an example.
With regards to the book review requests I've been sending...
Now you're talking about more than one object -- requests. That means now you add the s to make your sentence technically correct.
However, this is an extremely fine grammar point that most people ignore altogether. Regards and regard are used pretty much interchangeably. Use the one that sounds best to you for the particular sentence you're writing, and you should be okay. Remember that when you doubt your grammar, just read it out loud. If it sounds weird, it's probably wrong.