It won't pair.
It sends garbage.
It won't talk to your iPhone.

There is no shortage of free Arduino Bluetooth tutorials, and most of them work — right up until yours doesn’t. Then you are reading six of them at once, each confident, each contradicting the last, and none of them written for the state your bench is actually in. The one that told you to use 9600 doesn’t mention that your module ships at 38400. The one with the wiring photo is using a different module to the one in your hand, sold under the same three letters.

That is the gap this book was written into. It does not start from first principles and it does not start with a project. It starts from the symptom — the thing your bench is doing right now — and works backwards to the cause, then tells you the fix and how to prove it worked. The theory (Part 2) and the projects (Part 4) are there for after it works. Four boxes: your phone, the module, the Arduino, and the thing you want to move. Almost every failure lives in exactly one of them, and most of an evening is lost to not knowing which.

The numbers are checked against datasheets rather than against other tutorials, which matters more than it sounds: the most repeated baud rate for the HC-05 on the open web is simply wrong for a large share of the modules being sold, and it has been copied between articles for years. Every figure is drawn from the circuit, not sketched from memory, so the pin in the diagram is the pin on your board.

The single most useful page in the book is about naming. An HM-10 is a specific module from a specific manufacturer — and it has been copied, the copy has been copied, and the copy of the copy has been faked. All four are sold as “HM-10”, frequently in the same listing, at the same price. They share the same four pins and the same wiring, so the diagram you followed was right. What differs is which AT commands exist, what they answer, and whether the thing can be made to work at all.

This is why generic advice fails so reliably. “Send AT+NAME” is good advice for one of those four and meaningless for another. The book gives you a sixty-second test — two commands — that names what is in your hand before you change a single setting, and a fix encyclopedia that maps every symptom to the test that distinguishes the causes. If you take nothing else from it, take that: identify the module first, and most of the contradictions on the internet resolve themselves.

The same question decides which of the two products above is right for you. The book covers all four modules for diagnosis. The phone-as-a-remote part of the course needs a BLE module specifically, which is the one caveat worth knowing before you spend anything — and it is why it is stated above rather than after the checkout.