Which is better, learning from a book or digitally via iTheorie?
This depends entirely on your own preference; both methods have advantages and disadvantages. With a book, you can study anywhere, even in places where you don't have internet access. Some people also find it more comfortable to read from a paper book than from a phone or computer screen.
The advantage of digital learning via iTheorie is that you always learn the most recent course material. This course material is checked daily and updated if necessary. However, you always need an internet connection and it will use up your data allowance if you are not connected to Wi-Fi.