ReAction is just a set of classes for the Intuition BOOPSI framework, so the original Commodore documentation on BOOPSI (in the ROM Kernel Manual: Libraries) still holds good - although there are things that are done differently in ReAction:

• First and foremost, ReAction GUIs are based on an automatic layouting system while the original BOOPSI used absolute gadget positions and sizes (= it was the user's responsibility to adapt the GUI to window- or font size changes).

• Second, ReAction uses a different way to handle input events; see section 4.4 of my first ReAction tutorial.
  

ReAction currently lacks a comprehensive manual; you unfortunately have to study the old docs, the SDK examples and the scarce tutorials here and there.