Wolfram Rittmeyer (7) [Avatar] Offline
If I'm not mistaken the use of marble diagrams starts in chapter 4 without any introduction as to what they are. I think you should add one or two pages before to explain them.

Stick to one notation (apart from the glasses sample). In chapter 5 it's the style used for example for RxJS while in chapter 4 it's the one used for the operator documentation on reactivex.io, which is also used for the Javadocs of RxJava.

No big issue, but I think consistency helps.