This is the 56th most frequent Spanish word.
otro
"Otro" means "other" or "another" in English.
Here, 'otro' refers to an additional or different object ('libro') needed for a purpose, implying it is distinct from the already referenced or existing items.
Necesito comprar otro libro para el curso.
I need to buy another book for the course.
In this usage, 'otro' refers to a distinct or next item or subject in a sequence (another project).
Después de este proyecto, trabajaremos en otro.
After this project, we will work on another.
Here, 'otro' (in feminine form 'otra' to agree with 'película') indicates a choice or alternative, differentiating between entities.