Books look best with generous margins.
For SRA5 and A5 books we recommend 20mm margin on the top and both sides with 25mm on the bottom to accommodate the page number.
A4 size books can have margins up to 25mm with 30mm on the bottom. Do not make the margin in the gutter (inside of the page) of your book larger, we will do that if need be prior to proofing.
