Novelist & TV writer Stephen Gallagher (perhaps best known here for creating THE ELEVENTH HOUR) stumbed on tie-in writer John Garforth's blog:
Garforth wrote four Avengers novels for Panther Books in 1967. Two years earlier Hodder and Stoughton had put outDeadline and Dead Duck, two rather classy tie-ins written by Peter Leslie but with Patrick Macnee credited as their author… which seemed as transparent and ludicrous to my eleven-year-old self as it does now.
Both had their virtues. Leslie's books read like a literary source from which the show might have been adapted; Garforth's Panthers were shorter, racier, and had a more contemporary feel to them. All were true tie-ins as opposed to novelisations; which is to say, they were original works based on the series' characters, and not pre-existing scripts adapted into prose form.
Probably the oddest of Garforth’s works (which he mentions at his blog) was an adaptation of THE PALLISERS, which itself was an adaptation of a sequence of novels by Anthony Trollope. What Garforth did was convert a half-dozen very long novels about Victorian politics into a short bodice-ripper about forbidden passion among the titled set. Making it odder was that the American edition mentioned neither Trollope nor the TV series–there must have been at least a few people who came upon the book, recongnized its source, and mistook it for a piece of plagiarism.