These two books are wonderful for landscapes and comics, I always look back to both of these for reference, specially the landscape one :
Fantasy Landscapes & Cityscapes by Rob alexander : This book covers everything and is weird because is so thin!. It covers from tricks you can use depends on the medium you are most comfortable with, the importance of a sketchbook and use of references; perspective as well as depth. How to sketch common things like clouds,mountains and trees. It shows wonderful examples and tricks.
Comics and Sequential Art by Will Eisner : This guy is a legend, and if you want to learn how to make a GOOOD comic then you must have his book, I loved al his comics, he's very good in capturing the human body language, how to change the moods of an environment. He's a master at the comic art, he is mostly known for his comic "The spirit" I was more fan of his other works about everyday people like "New york: Living in the Big City".
Well here are the peeks: