"The book is expensive."

Translation: Cartea e scumpă.

[SophieKostovski]

when would you use "e" vs "este?"

[potestasity]

They're perfectly interchangeable in meaning. The key difference is that "e" is shorter by one syllable, and thus preferred in day-to-day speech. One may alternate between them for controlling the rhythm of one's speech, e.g. in poetry.
Also it's a common sentiment that "longer" = "more formal", so I guess some would only use "este" in formal contexts, but I find this very silly.