They are totally different books, written for different user levels.
The On Excel book assumes that you are an Excel guru. It presents idea after idea after idea without a lot of explanation about why things work - the book assumes that you can figure out why something works once you see it. Even hard-core Excel users are amazed at what they find in this book. There are tips in here that were never seen elsewhere before the book was published.
Learn Excel is written for the average Excel user. It is long on explanations of how things work and why they work and what might go wrong. If you are already an Excel guru, it will seem overly simplistic to you. However, if you are like the average Exceller, you will learn real life tips that can save you hours per week when doing real life jobs. The 277 Excel mysteries solved are mostly real life questions that were asked during the Excel Power Seminars. These are students who felt they were smarter-than-your average Excel user, but had not figured out how to use many techniques in Excel. Tips span user levels from beginner to intermediate.
