I have to wonder what connections there may be here between O'Keefe and Foster. They were born within a year of each other in the 1890s, and died within a somewhat close time of each other in the 1980s, both working as illustrators turned cartoonists during those long years of the 20th century.
O'Keeffe's style in this book looks somewhat like Foster's, even with Valiant's pageboy haircut on display. Yet O'Keeffe's cartoon style that he used in Dick's Adventures, years later (seen at the bottom of this post), looks (to me) very similar to the comic strip style that John Cullen Murphy used when he replaced Foster on the Prince Valiant strip.
O'Keeffe and Foster seemed to cross paths in Chicago and elsewhere in their early illustration careers. O'Keefe, Foster and Murphy all worked for the King Features Syndicate (owned and guided by William Randolph Hearst), during a concurrent period in the 1950s, Murphy drawing Big Ben Bolt.
To further mystify my curiosity, the fellow who wrote Dick's Adventures was Max Trell, and that same Max Trell wrote the text for Hal Foster's hard cover Hastings House adaptations of Prince Valiant in the 1950s.
I've casually tried to find an official connection between O'Keeffe and Foster, such as a friendship or sitting at the same table at the NCS gatherings, or anything—to no avail. There may be no connection whatsoever, but if anyone knows more of the backstory alluded to here, please do let me know. The most curious thing to me is the similarity of this book's illustrations to the later Prince Valiant, not saying whether Foster was influenced or not.
Neil O'Keeffe — In the Court of King Arthur — cover
This and most other black and white illustrations in the book (most not shown) seem stylistically to belong to another hand.
The one below initialed G.G.
Below, O'Keeffe's cartooning style on Dick's Adventures that I feel is reminiscent to John Cullen Murphy's style on Prince Valiant.