The Tips give a nifty table of declensions that..... say nothing about the "is" ending (as on litteris).
Wiktionary says it's 1st fem, a dative or ablative plural. True?
And why dative, not accusative? It seems to me that Latin literature is directly the object that we're studying.
It's the direct object in English. In Latin, "studere" more accurately parses out as "to dedicate oneself to", therefore it takes the dative rather than the accusative.
And yes, "littera" is a feminine noun which literally means "letter". In the plural it can mean "literature" (we still use that turn of phrase to this day: "a man of letters" for example)