The eBay Price Guide is a big book, over 500 pages, including the appendix, of pure information. Wilkinson has done a service to anyone who sells (or buys) on eBay with the amount of statistics, closing prices, etc. that she has assembled in this book.
Every single category that is offered on eBay is included in the book. Everything from Antiques - Classical - Egyptian to Video Games - Wholesale Lots - Other. Wilkinson lists the average price of items sold in each category, then lists the sales of high dollar items in the category along with the sales of items that were closer to the average price. You can see categories that are trending high, and what items are selling for really good money. You'll get an idea of what to expect when you list that US Marines 230th Anniversary Silver Dollar Proof that Uncle Ed gave you for Christmas a few years ago ($50, about average for its category).
The problem with a book like this is that it is quickly out of date. The numbers that are listed should be considered indicators of trends, not indications of what you can expect today on eBay. So your silver dollar might only get you $30, or you might end up attracting a bidding war between two retired Marines who bid the thing up to $150. Or you might get no bids on it at all. But the book gives you trends, so that if you are trying to figure out what to sell, you can look in the book and see what things are selling in each category.
Wilkinson also begins each 'chapter' (the listings for each category) with some advice about buying and selling in that category. These introductions are probably the most valuable parts of the book, as they can help you to interpret the data that is compiled in the rest of the chapter.