You can't learn C++, or any programming language, or anything, in 21 days...
of course not. but it's like the ghost shooter tutorial, it lets you dive in and get a good overview of the basics, and a basic grasp of a wide array of concepts. from there you can delve deeper into that which is unclear, or that which interests you most. it isn't so much the speed of the book, it's the conciseness. I suppose it depends on the learner, but I find learning programming more enjoyable when I first see the whole picture, and then I lean in for a closer look from there, rather than getting tiny pieces of the picture exposed, while being blind to the rest. especially since at the time I decided to really learn c++ was to start making plugins for construct. i think it's a much more fun learning platform than the text console
but ashley's right, definitely don't go into it trying to learn it as fast as possible. learn it as thoroughly as possible.