These are more generically known as ‘mass market paperbacks’ - and are sadly basically extinct:

https://www.yahoo.com/lifestyle/articles/end-mass-market-pap...

Last year, ReaderLink, the country's largest distributor of books to airports, pharmacies and big-box stores, announced that it would no longer carry mass markets. The books can still be found in some places, but "as a format, I would say it's pretty much over," Ivan Held, the president of publishing imprints Putnam, Dutton and Berkley, said to the Times.

I too mourn them - much more of a convenient size than the now ubiquitous ‘trade paperback’ - but we’re obviously in a small minority.