In my beginners Norwegian, I learnt that “ei jente” was the correct translation of “a girl.” In Lesson 2, however, “en jente” is described for “a girl.” Which is the correct form, please?
Helllo from Norway. 
Both forms are accepted.
Other examples:
ei rotte / en rotte (a rat)
ei bok / en bok (a book)
ei klokke / en klokke (a clock)
ei dronning / en dronning (a queen)
etc.
Basically all feminine nouns (which use “ei”) can also be inflicted with the masculine conjugations.
In addition to this, while it’s technically not correct, for feminine nouns Norwegians will often use the masculine inflection for the indefinite form (en bok) and then move onto using the feminine inflection for the definite form (boka). Such inconsistency is frowned upon in very formal writing, but is is very common in informal writings and in speech.
So we’ll often say “en jente” for the indefinite. However “jenten” (definite) sounds weird so most people will say “jenta” instead, so then you get this mix.
Bear in mind, it is only feminine nouns that have this flexibility. Masculine and neuter nouns must be conjugated using their inherent forms.
Hope that makes it a bit more clear. If you find this confusing, you can just stick to using the normal feminine inflections for the feminine nouns and you’ll be just fine!
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Thank you.