With every library we donate, we work very closely with the students, teachers, and even parents to make sure we are giving them books that are relevant and needed. An intuitive step in this process is ensuring that they submit requests to us, whereby both the students and teachers have had a chance to voice their wish list.
Below is an example of a request list from Mwendakulima Secondary School, which is one of the projects we are currently working on, in Kahama, Tanzania. This request was for English (they submitted similar sheets for every subject).
Although we always make it clear at the beginning of any partnership that we do not donate textbooks, we inevitably find them amongst the list of requested books. However, our mandate remains strictly library books. 
