I am not entirely sure if we are allowed to ask such questions here but I was wondering if anyone could suggest me a real introductory level textbook on modal logic
Chellas, Brian F., Modal logic. An introduction, Cambridge etc.: Cambridge University Press. XII, 295 p. hbk: £ 17.50; pbk: £ 6.50 (1980). ZBL0431.03009.
This is a very carefully written book. It goes into the beautiful completeness theorems for S5, S4 etc. but it starts out just assuming you know some propositional logic.