I am a very experienced user of Adobe Photoshop and have what I understand is the last standalone version of the product in CS6. Being retired and having given copious amounts of money over the years to Adobe for products and upgrades, I have no intention of joining their "Cloud" offering. Eventually my CS6 may become less functional so time to look for an alternative and the GIMP offers most. The key to PS is the use of layers so I needed a guide just to understand any differences between the 'PS Way' and the 'GIMP Way' of handling layers so this was an excellent 'book' for my purposes. To make it more convenient, I bought it as a Kindle ebook to load on my netbook to refer to whilst I worked on the GIMP software using a full size PC.
One final thought: Because I have been using 'layers' for years and that is why I bought this, whether someone new to the concept would get 'instant enlightenment' from this book I don't know for sure but I think they would, it is well written.