This is the 137th most frequent Icelandic word.
önnur
"Önnur" means "other" or "another" in English.
Here, 'aðrar' refers to OTHER books, emphasizing a substitution or competitive characteristic.
Ég hef þegar lesið þessa bók, en mig langar að lesa einhverjar aðrar.
I have already read this book, but I want to read some others.
In this case, 'önnur' indicates being OTHER in terms of contrast or distinction.
Ein stígvél var of lítil, og hin hitt var önnur.
One boot was too small, and the other was different.
The usage of 'önnur' denotes all others in the set of tasks, distinguishing them from the current one.