Are you making an ebook or printed one?
If latter, then you should check what kind of technical requirements (resolution etc) the printer has set for images/media, and use only photos qualifying the criteria (otherwise the results will look pretty crappy when printed).
And are you seeking images of mint or used (cancelled) stamps, or do you qualify both?
And some details of the project (what's the title, who's publishing, what's the topic/approach of book etc) would be also nice to know. Out there are lots of folks who 'misuse' the photos we collectors so generally provide & share.
Quote:
I am aware but wonder how to find the author of each stamp....
A somewhat common practice is to put 'The designs of stamps are copyright of issuing postal authorities and their designers' clause and rely it falls to fair use. Of course if it is good practice to approach each postal service (marketing/philatelic dept) separately (with email) and ask for permission - usually they have no objections as it is free PR for their 'products'. This page here has got lots of good info on stamp copyrights of various countries:
https://commons.wikimedia.org/wiki/...ublic_domain-k-