"This book is short."
With some imagination you can see the kanji as a little guy failing to reach the "you must be this tall to..." sign.
この is only used with a noun immediately following it; これ is a standalone pronoun. It's the difference between "[This] is my book." and "[This] book is interesting."
You use この when you mention the noun or the thing. これ is used when you don't mention the noun.