At various times, and for various reasons different Sasek postcards have been produced. There are the Paris postcards, and at least two sets of postcards produced from the books: This is Hong Kong and This is the United Nations, in both cases to accompany stamps of the same design.
In 1985 (maybe earlier) and 1994 Editions Hazan published at least seven postcards featuring Sasek views of Paris landmarks. The postcards were sold as regular postcards as well as glued/mounted on a folded card stock. In 1995 the same were printed as large folded cards. The cards were numbered so are obviously part of a series, but it is not clear how many there are, or what else was in the series. All cards have Sasek's initials as part of the painting, with one (L'Opera) also having the numbers "'58", presumably for 1958 when it was painted. This makes sense as This is Paris, the first book in the This is series, was published in 1959. The pictures are similar to some of the views in the book, but are not taken from it.
See the cards:
In March 2003 the United Nations Postal Administration issued two sets of 5 postcards in the denominations of 23¢ and 70¢, and two 37¢ pre-stamped envelopes with illustrations from Sasek's This is the United Nations.
Better pictures will be added shortly.
Postcards from This is Hong Kong were also produced which were similar to the UN ones. I think they were also to commemorate stamps that were produced (in Hong Kong presumably). From what I remember they were very nice, so if anyone has more details please get in touch.