James M. Hutchison has an extensive collection of stamps that feature literary topics, and part of it can be seen at http://faculty.citadel.edu/hutchisson/Pages/Stamps%20Revised.htm. He states that “Among countries Great Britain by far and away leads the rest in terms of special stamp issues that feature literary topics.”
If, however, you are interested in American literary postage stamps, why not take this quiz of 30 American postage stamps with a literary theme: https://www.sporcle.com/games/hatefulmissy/writers-on-us-postage-stamps-picture-click.
If you just want to view some literary postage stamps from around the world, check out https://www.pinterest.ca/threebookends/literary-stamps/?lp=true.
Of course, postage stamps are often issued after a writer’s death. See http://lithub.com/17-living-writers-currently-immortalized-on-stamps/ for some living authors who have been immortalized on postage stamps.