In order to understand Barry Letts' three efforts as a Doctor Who novelist (The Ghosts of N-Space, Deadly Reunion, Island of Death), you first need to understand the period in which they were written. The Wilderness Years, as they've become known, was an era in which Doctor Who had no presence on television--the only way to experience the show was through an alternative medium such as books, comics or radio. However, there was still a large audience for Doctor Who stories.
More accurately, there were still two large audiences for Doctor Who stories. On the one hand, there were a large number of fans who felt like the series had been cancelled just as it was hitting its stride and reinventing itself, and they best loved those stories that pushed the boundaries of what Doctor Who had been doing over the last few years of its televised existence. (Doctor Who stories that fell into this category were usually called "rad".) On the other hand, there were a large number of fans who felt like the series had gotten too far away from its roots, and preferred stories that returned to a storytelling style more in keeping with the older episodes. (Doctor Who stories that fell into this category were called "trad".)
Barry Letts, as a former writer, producer and director for the show, was much beloved as an elder statesman figure by the fans who enjoyed "trad" Doctor Who. His heyday, the Jon Pertwee era, was to some the epitome of what the series did best. This audience was present in sufficient numbers to get Letts, along with Jon Pertwee and Lis Sladen, to reprise their roles for two radio plays on the BBC's radio network written and directed by Letts. As was generally the case, Virgin Books (who were starved for new material during the Wilderness Years) adapted the first one as a standalone novel, and the second one as part of their Missing Adventures series.
This second novel, 'The Ghosts of N-Space', proved to be something of a sticking point. Because although there were a lot of "rad" fans, and a lot of "trad" fans, they didn't necessarily congregate in the same places or look to the same sources for their Doctor Who enjoyment. And Virgin Books, with its New and Missing Adventures, was definitely the home at the time of "rad" Doctor Who. 'The Ghosts of N-Space' felt very out of place next to the New Adventures, and even somewhat out of place along the more overtly nostalgic Missing Adventures. Later, when he published 'Deadly Reunion' (with Terrance Dicks, his script editor during the Pertwee era) and 'Island of Death', the situation had in some ways gotten worse--"trad" fans had moved to the Big Finish audios, which featured the original actors reprising their parts, and Letts' novels were even less welcome. Because fundamentally, Barry Letts hadn't really changed anything about his style since the early 1970s.
His novels fell very much into the "Boy's Own" style of pulp adventures, with very thin and very straightforward plots driven by villains without much motivation beyond the desire for power for its own sake. His prose could best be described as similar to the Target novels of the 1970s, serving mainly to transcribe the events of the story with very little in the way of the stylistic flourishes that were in vogue at the time. Most of the characterization in his books came from recreating the actors' performances, although Letts did introduce a new companion in the form of Jeremy Fitzoliver. You could say that this was a radical thing to do, as Jeremy didn't fit into the mold of most companions; he wasn't very bright, or very brave, or very interesting, or even tolerable to be around for more than a few minutes. But the same qualities that made him somewhat experimental as a companion made him also pretty difficult to stomach reading about.
In short, Letts was primarily a producer and director who was asked to write, in order to please an audience that primarily wasn't paying attention to novels. As a result, the novel-reading audience that did pick up Letts' books was anything but charitable to his old-fashioned style. This isn't to say that his novels are terrible; in fact, divorced from the expectations that were placed upon them during the Wilderness Years, they're quite readable, if fairly forgettable, action-adventure novels. If they'd come out during the period they were set in, Barry Letts' tenure as producer of Doctor Who, they'd probably be quite fondly remembered. But Doctor Who had moved with the times, and it's hard not to feel as if Letts might not have been better off serving as a beloved elder statesman than as a modern author.