Translation:Read both books before the next class.
Accusative feminine (it could also be nominative, but not in this sentence). Masculine form is «о́ба». They look strange because they were dual in the past; now, we don't have dual number, but this word retained its old ending so it looks different from other pronouns.
Thanks. I forgot about the dual. So it is like два then? And is there also a neuter form?
Actually, it's declined like «двое» and not like «два» (althoug they're definitely related). You can see the declension table in the Wiktionary.
"Read both books before the next lesson"
This is counted as incorrect. Why isn't Russian as intuitive as the other languages?