Megasthenes’ Indica refer the country as India, Ἰνδία.
Ctesias’ earlier Indica refers to the country as Indikē, Ἰνδική, as does the even earlier Herodotus.
The former is a country name, “Indus-land”. The latter is a feminine singular adjective, the Indus-ian (country). The same adjective is used, in the neuter plural, in the books’ name: Indika, the Indus-ian (stuff).