Races & Classes provides an overview of the design philosophy WotC has put into the development of 4e, as the name of the book suggests; primarily in the department of races and classes.
This book is NOT a preview of the 4th Edition rules. You will not see any hard "crunch" in it. This is more of a "the making of" type product and is billed as such.
The art in the book is simply amazing. I've disliked much of WotC's art for years. If this book is a preview of the look they will maintain for 4e, I'm impressed.
It gives an in-depth overview of 7 races: Humans, Dragonborn, Dwarves, Eladrins, Elves, Halflings, and Tieflings. These overviews give some interesting "fluff" about each race, setting the scene for them. After the fluff, there are generally design notes on each race telling the reader what it is they are trying to accomplish and the reasoning behind it.
The same treatment of the classes is given; fluff followed by the "why" of what 4e is trying to accomplish.
My primary complaint with the book is it's price. $19.95 seems a bit steep for this type of product. Although I suppose the fact that I am willing to buy the next preview book as well means I don't consider the price to be over the top. I'd be more pleased with a $10-$15 range though.
If you are interested in design philosophy and seeing what goes into the making of the game, this is a good buy. If you are hoping for concrete peeks into the rules system, wait for the SRD or core books.