"They have been decreasing," he said, adding it appeared to be a generational thing.
School librarian and sale co-ordinator Vicki Clarke echoed that.
"There's a missing generation we no longer see at the sale."
It revolved around people from 16 into the early 30s.
"I'd like to think they're getting their reading online but that might be optimistic," Clarke said.
Murfitt said there had been a shift in what was now donated, and while they were once prolific there were very few encyclopedias any more.
"All that sort of thing has gone online."
Clarke said having to call an end to the annual book sale was "heartbreaking" as it had seen many donated books end up on the library shelves, in classrooms "or pressed into the hands of eager young readers".
The sale's key reward through the years was keeping the community reading but there was also the many positive links the sale fostered between the school and the sale's supporters, Clarke said.
It was a project initiated by the school's Parent's League in 1989 and the sales generated enormous goodwill and built bonds between parents, college staff and service groups such as Rotary, which has helped set up the sale tables for many years.
Prior to the June sale each year, a team of parents and the parents of former students would spend hours each week for months sorting many thousands of books.
"It's laborious but very social," Clarke said.
Murfitt said he would like to see the sale go out in style with plenty of donations and plenty of people heading along on the weekend of June 8 and 9.
Books can be dropped at the William Colenso College office on Arnold St, and if they needed to be collected people could phone the school office on 831 0180.
Clarke said ideally all donations needed to arrive before the end of April to allow time for sorting.
"This is your last chance to clear your shelves and support a great cause."
She said judging by the type of books donated in the past, Napier people appeared to be fine cooks, good golfers, well travelled, romantic and slightly funny.