What are some easier books for studying martingale?
They are defined to be comprehensive but easier than Roger and William's martingale book. For example, to study Q and F martingales?
It should cover continuous-time martingale, stochastic integrals and most other basic topics for martingle
I think it's pretty hard to find a book which covers martingale theory; usually, books either give just an introduction or they focus on one particular aspect of martingale theory. I'll list some books which might be of interest and sketch (roughly) which parts they cover:
And for the sake of completeness (... since I think that the book is comprehensive, but hardly an easy read):