Well, generally speaking, if you start with some subject, you have no idea what's important and in which order to learn things. You can often do it yourself and just start somewhere. But it'll be more effective and quicker if you start with the basics first and then the details. And let someone who knows this stuff simply tell you.

Also sometimes you need to know some concepts to be able to understand something. For examples the maths, trigonometry before trying to program a robot arm that needs to move somewhere. If someone figured this out for you, you'll be faced with the assignments in the correct order and you'll have the background to start learning this topic. If they're a good teacher, this will be most effective and also make sure to keep you motivated. In this example you could figure it out yourself and see what background knowledge you now need to learn. But once it gets more complex, it won't be that easy. For example with programming, knowing about heap, stack, harvard architecture and memory protection will make it obvious for you why you can't change a C string, why you can't grow and shrink some objects but you can do it with others. Why some of your code just runs slow. And you probably won't know to look up 'heap' while looking at your code to deal with json-objects. You're more likely to memorize when to write 'new' something and learn lots of exceptions individually and not understand some things at all. While someone with a 'traditional' education will know the concept behind that.

And sometimes you'll get lost in details if you do it on your own and waste quite some time. Or you'll miss out on some important fact, because this case didn't occur in your practical experience.

But that's only part of the truth. People with a 'traditional' educations also sometimes don't know stuff, miss out on concepts and struggle with the same things. And self-taught people often have other beneficial character traits. And you should never underestimate their level of motivation and dedication to a subject. And you can learn how to learn. I - personally - think it's more important to do something with love and dedication, than to do it 100% the right way. And to have fun. But since you asked why it's better to read the book first, these are some reasons to do it that way round. And as I said, don't skip out on the actual experience. There are people who only have theoretical knowledge. And it's a difference to hypothetically know how something is done, and actually knowing about all the consequences and caveats in practice, because you've already done it 3 times.

I don't have any book recommendations. I also taught myself lots of stuff. Read computer magazines, distro-hopped for quite some time. Read most of the documentation while dealing with some problems during the years. Deleted all the files for shits and giggles and learned about the filesystem hierarchy on Linux. Ran a webserver with lots of services. Studied computer science... Read a few books from the library... I'm sure other people here have good recommendations on what to read as of today. If not, go to the library or bookstore and have a look at the books, see if you like the style of some book. Maybe a university or a free software organization has some course available or some bibliography.