English expression of concrete fixed plans of the future

I have introduced the English grammar book which I'm using now. It's "Grammar in use". I call it "Murphy book". (Because it was written by Raymond Murphy.)

The Murphy book says in its section of the future expression that when you express plans which have been already scheduled concretely you use present continuous.
For example;

I'm not working tomorrow.

She is arriving at the airport at 8:30.

But according to my previous English grammar book, simple present is used to express fixed plan of the future. This book's example sentence is;

He leaves for San Francisco next Friday.

Hmm...

For now, I concentrate on the Murphy book.

Since I start studying with the Murphy book, I have been able to clear some things which had been obscure for a long time. I rely on this book.

The previous book was not so bad though.