I stopped reading Nield's other RxJava book to check out this one, which is much the same (as one might expect). It's somewhat dissatisfying, in the sense that the particular features and functions of the RxJava library are not as interesting or important as the concepts behind RxJava, and they're only slightly dressed up here with JavaFX stuff.
This isn't to say that this is bad, though: Nield clearly knows where the interesting points are, and he calls your attention to them so even if you haven't mastered them, you know what to look up and experiment with.
Also: Free. You can download this in PDF or read it online. I've paid plenty for much worse.
It would be much more interesting to have a full book where you built up an app using RxJavaFx, but I'm not even sure the book market can bear that sort of thing these days.