Don't learn an API by trying to cram everything, you don't have to be able to recite every function and every class available, that's just madness. Also, don't worry about using the 'correct book', or the 'correct tutorial', because these should only be reference materials and not absolute guides to learning and understanding an API. Learn by doing. Read up on some tutorials and try to recreate what they achieve by yourself, while keeping an API reference close so you can look up which parts of the API you should use and how they should be used. When you use these parts of the API a couple of times you'll have no problem remembering what they do and how they should be used, maybe you won't know the exact details of which parameters a certain function expects, but that's absolutely no problem as that's what those books and online references are for. If you're serious about programming you'll encounter tons of APIs in your career, and there's no way you're going to be able to remember or learn every single detail of each API you're ever going to work with, so get comfortable with using the available documentation as reference material.

Whenever you 'feel' like this, just remember back to the first time you programmed or your first experience with OOP and all the things that were confusion, overtime things made sense and you understood stuff, the same will happen with DirectX, however do note the learning curve with any lower level library is steep but once you have gotten use to it, it can become a lot of fun and you will accept the challenges with more confidence. That book is ok for beginners I guess, but from personal experience anything with a skull hand at the front is terrible, and I am pretty sure that book doesnt cover major areas that probably should be covered in a beginners book. Have a look at Introduction to DirectX11 - Frank Luna