Fact is that there was never a book on digital scopes written by anybody.
There is a book in german avaible and it´s mainly a kind of manual of the authors scope...

A good useful book could build like this:
- Short explanation for what a scope good for is.
- Block diagrams of analog and digital scope, short explanation of the differences between the two.
- Then taking the single blocks of the DSO diagram as chapters where the functions will be explained.
- After this some measure examples to understand how the whole blocks are working together.
- Additional functions explanations like FFT, Decoding, Measures, XY mode and so on