The Girl from U.N.C.L.E. which only lasted a single season (1966-67) gave birth to no less than five original novels, although for some reason three of them were only ever published in Britain.
The Birds of a Feather Affair by Michael Avallone appeared in 1966. It’s notable mostly for differing quite sharply from the television series when it comes to tone. Here's the link to my review on Cult TV Lounge.