Manuscripts are original versions of books which have the writer's scribbles and corrections all over them.
Pullman - who writes in his garden shed with a pen and no computer - told BBC Radio 4 they are interesting because you can see how the writers work on books.
When books are best-sellers, manuscripts become really valuable. But most are snapped up by US universities and libraries.
No Dark Materials
Unfortunately the trunk-loads of manuscripts won't include those from the Dark Materials trilogy. He classes those as adult books!
The last in the trilogy, The Amber Spyglass, won the prestigious Whitbread Book of the Year award in 2001.