in 1) the meaning is that it was happening (пополнялась книгами), it is important that it was happening
in 2) the RESULT is important, you underline that there is a certain RESULT (пополнилась книгами) and this result is a matter of interest (hence, св is used)

the same holds true about В начале лета выпускники школ (сдавали/сдали) итоговые экзамены и (получали/получили) аттестаты зрелости.

you can say В начале лета выпускники школ сдавали итоговые экзамены и получали аттестаты зрелости. - this was just happening generally to all students and all schools

but you can also say: В начале лета выпускники школ сдали итоговые экзамены и получили аттестаты зрелости. - here you stress the RESULT (that they held exams and they got certificates)