"Ace Books published its K series of books starting in 1959 and continuing to about 1968. The K series was priced originally at 35 cents, and later at 50 cents.
Some titles are missing from this list, and may not exist. The missing serial numbers are 130, 131, 135, 165, 186, 253, 256, 270, 274, and 302. Note also that there appear to be two books with number K-228, according to book listings; neither has been seen for this article.