The sentence is written somewhat clumsily. But here's what it means:
In order to learn to make games with this tool …
You may need to change some habits and expectations (that you acquired from experience with other tools).
That (need to change habits and expectations) is a reason why you might not benefit from reading this book.
But, I do not see any additional reasons why you would not benefit from reading this book.
In the sentence, "besides that" means "other than the (possible) need to change habits and expectations". The word "that" refers to the need to change habits and expectations. The word "besides" means "other than, apart from, in addition to".
Here's why the writing is clumsy. The fact that you may need to change habits and expectations is a very weak reason to think that you wouldn't benefit from reading the book. If I understand it correctly, the need to change habits and expectations results from the nature of the tool, not from the book. So, the meaning of "besides that" is a little hard to figure out. To figure it out, you need to understand that the sentence is talking about reasons why you wouldn't benefit from reading the book, which doesn't come until later. Also, the sentence actually gives another reason why you might not benefit from reading the book: if your goal is not to make games using the tool!
It would be simpler and clearer to write:
As you learn to use this tool to make games, you will likely need to change some old habits and expectations.
and then explain something about that change in habits and expectations. It might be even clearer to omit the sentence altogether. There's no need to suggest that the reader wouldn't benefit from reading the book.