For those who are interested in synchronous reactive programming and speak French, Gérard Berry gave lectures about it (and time-related topics in general) at Collège de France, along with other teachers including M. Pouzet. Videos are publicly available, check out:
- Gérard Berry: "Real time programming: Special purpose or general purpose languages"
For the semantics, I like this paper focusing on abortion (for me, the most expressive construct of synchronous languages):
- Gérard Berry: "Preemption in Concurrent Systems"
There's also a well-known survey:
- Albert Benveniste: "The synchronous languages 12 years later"
This one is about bridging the gap between synchronous and asynchronous, something like Esterel+CSP:
- Gérard Berry: "Communicating reactive processes"
For a modern take (besides Céu), check ReactiveML:
- Louis Mandel: "ReactiveML: a reactive extension to ML"
You can also search for papers from professors Stephen A. Edwards and Reinhard von Hanxleden which still work actively on the subject.