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.
Check out Stephen's appreciation and, of course, Garforth's blog.