Books about modal logic? I've just approached modal logic reading "An Introduction to Non-Classical Logic" of Graham Priest.
I am looking for some books that treat this argument in a more extensive way than the book I am reading.
I am especially interested in the philosophical  side of modal logic.
Can you suggest me some books?
 A: Here are a few recommendations for you, all with an emphasis on the philosophical aspects of modal logic:
"Modal Logic for Philosophers" (Garson, 2006).
"Modal Logics and Philosophy" (Girle, 2000).
"Handbook of Philosophical Logic, Volume 14" (Gabbay and Geunthener eds, 2007).
"Modal Logic: 
An Introduction to Its Syntax and Semantics" (Cocchiarella and Freund, 2008)
I hope these help.
A: I personally learned modal logic from Chellas's Modal Logic: An Introduction, but a more modern treatment in-line with current interests in modal logic is van Benthem's Modal Logic for Open Minds. Other good introductions include Modal Logic: An Introduction to its Syntax and Semantics, Cresswell & Hughes's A New Introduction to Modal Logic, and Beall & van Fraassen's Possibilities and Paradox: An Introduction to Modal and Many-Valued Logic. Also, take a look at the SEP article on Modal Logic (SEP is in general a great place to skim through the bibliographies for references on various philosophical topics, including philosophical and mathematical logic).
A: https://www.elsevier.com/books/handbook-of-modal-logic/blackburn/978-0-444-51690-9
Patrick Blackburn Johan van Benthem Frank Wolte
https://www.cambridge.org/core/books/modal-logic/F7CDB0A265026BF05EAD1091A47FCF5B#
Patrick Blackburn, Maarten de Rijke, Universiteit van Amsterdam , Yde Venema, Universiteit van Amsterdam
https://link.springer.com/chapter/10.1007/978-1-4020-5587-4_5
Johan van Benthem, Guram Bezhanishvili
