This book is perfect for some, but not the best choice for most Mini owners.
If you own a Mini, you're probably not impoverished, and you're probably not keen on cutting corners and taking risks with your car just to save a few bucks. You might not be a do-it-yourselfer at all -- in which case you don't need this book at all. This is the book for you if you're someone who wants to do a only few basic jobs on your car, like oil and filter changes, and maybe you're curious about more advanced work, not that you'd do it yourself but you want to read about it. If that's you, then the Haynes might be a perfect fit.
But the safe choice is to start with the Bentley -- "MINI Cooper, MINI Cooper S, Convertible" (ISBN 9780837616391) by Bentley Publishers. Given that the amount of money you could save (with the right manual) in labor and parts could easily run in the hundreds or even thousands of dollars, the extra $60 or so is trivial. One major repair at the Mini dealer can run you $1000, easily. If you think there's any chance you're going to dig deeper than routine maintenance, then you're likely going to end up needing more and so the low cost of the Haynes will be a false economy, and a waste of money. And for that you need the Bentley.
While the Haynes is only 224 pages and only costs about $20 to $25 or so used, the Bentley runs 1400+ pages and costs $80 to $90. It is not perfect -- not by a long shot! -- but it covers far, far more than the Haynes, in far more detail. The Bentley is nonetheless frustrating because you will find yourself flipping around between chapters to finish a major job, and it doesn't tell you of the many obvious and easily obtainable alternatives to the expensive BMW tools that are "required" for most major jobs. For that you need mechanical experience, guile, and the Internet. Still, it is the best and most complete book of the two, and the price is worth it.
The other person who might want the Haynes book is the even more serious do-it-yourselfer who already has the Bentley book, and who is looking for even more. Perhaps alternative approaches to certain tricky jobs, like bleeding the brakes without getting air in the ABS unit. If that's you, then go ahead and spring for the Haynes. It *does* indeed have alternative brake bleeding, and slightly different steps suggested for big jobs like removing your engine or transmission. Not necessarily *better* methods, but food for thought.
Just so you know, *neither* book tells you what to do if your brake reservoir does run dry and air gets all up in your ABS system. The Bentley is silent, and Haynes says get the car to your dealer and have them fix it with some unnamed "special electronic equipment," whatever that is.