As a contributor to the new book Investing an Uncertain Economy for Dummies, I received the right to post online a PDF version of the chapter I wrote, Diversifying Your Stock Portfolio by Size. The chapter explains the differences between large, mid and small caps and how including a bit of each in your portfolio can help overall performance.
Here's a link to the chapter. It will give you a flavor of what the overall book is like and maybe inspire you to purchase it.
Just published in October, the book was a collaborative effort by financial planners like myself who belong to the Garrett Planning Network. In addition to writing a chapter, I served on the editing team that coordinated the project.