Book of folded sheets (5)
I believe the answer is:
folio
'book of folded sheets' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for folio that I've seen before include "Sheet folded to form two leaves in book - page number" , "Four pages" , "Folded sheet - or book made from such" , "outsize book" , "Book made of folded sheets" .)