By and large, the continuations of the James Bond series written by Ian Fleming's successors get very short shrift, often rating no more than a few pages in studies of the series--like those by Jeremy Black, or Simon Winder.
And some writers get shorter shrift than others, John Gardner in particular seeming neglected, relative to his contributions. This is partly a matter of timing. He was neither the first (like Kingsley Amis), nor, of course, has he been the last (in the nineteen years since Gardner's last, five different writers have tackled the series, often in sharply different ways), which by itself makes him more easily forgotten.1 Additionally, in contrast with the prestige some of those authors enjoyed due to their more "literary" work (Amis in particular), Gardner was seen as principally a genre writer.
However, some of this has to do with more than timing and bias. Gardner's enthusiasm for the books has been open to question. His career as a novelist began with a self-described "piss-take" on Bond in the very funny parody Boysie Oakes novel The Liquidator, and he later admitted in an interview that he never really cared much for the character. And those cognizant of his other work may be all the more dismayed when comparing even his better Bond books with the sheer verve Gardner displayed in writing a book like The Liquidator, or what were by his admission his favorite novels, the books of the Moriarty series (in which he displayed more verve continuing an entirely different character).
It does not help that it was not a case of a writer growing into a task and going out strong, but rather the reverse. (Licence Renewed, for example, was a deft blend of Ian Fleming with the cinematic Bond, which made for one of the overall series' most satisfying action-adventures--but he went in other directions with later installments, and the last entries show clear signs of exhaustion.)
Still, if the overall quality of the output was not all that might have been hoped for, many entries did have their pleasures, and even those books that drew more ambivalent responses enjoy the interest of novelty--like the Bond-meets-Top Gun of Win, Lose or Die. And even if they are apocrypha rather than canon, sheer mass lends them an additional significance. No one, not even Fleming himself, spent more time on the series or produced as many books in it as Gardner's sixteen years, in which he published fourteen original novels (and two novelizations of Bond films, those of Licence to Kill and Goldeneye respectively).
A really complete appreciation of the series requires that they be taken into account, and indeed, in my two recent books--The Forgotten James Bond and James Bond's Evolution--I have made a point of doing just that.
Just Out. . . (James Bond's Evolution)
Just Out . . . (The Forgotten James Bond)
My Posts on James Bond