The question of what were the best "post-Fleming" James Bond novels is a complex one. Just what standard ought we to apply?
Are we looking for more fiction just like what Ian Fleming produced? If so, then Kingsley Amis' Colonel Sun comes closest--helped by the fact that Amis had prior, and direct experience of meeting Fleming, helping prepare his own fiction for publication (reviewing the galleys of The Man with the Golden Gun).*
It helped, too, that he produced his book just a few years after Fleming wrote his last word, before his work dated nearly so much as it was to do later. Indeed, every writer who came after Amis had little choice but to downplay Fleming's attitudes (even Sebastian Faulks, writing "as Ian Fleming"), while changes in the broader political context nullified many an old concern. (What need for imperial policing when there is no empire? And did the hand-wringing over the welfare state unsurprising in the age of Macmillan make any sense in the age of Thatcher?)
Moreover, those unavoidable changes apart, the fact remained that, as a writer aiming not for pulpy adventure but "thrillers designed to be read as literature," Fleming was prone to an oblique narrative style, and character drama. And with only a few exceptions, the result has been that his tales seem slow and lacking in action from the perspective of those accustomed to today's thrillers. (Or at any rate, that was how this reader of today's thrillers found them when first trying Fleming.) That Fleming tends toward a bleakly irony view of life does not necessarily make this more appealing.
By and large, today's reader is likely to prefer something more accessible and less literary, brisker and more action-packed; something more like the Bond films which are the basis for almost everyone's perception of Bond today, even those of us who eventually read the books. And indeed, most of the post-Fleming books have something of this about them. However, if the standard is that of a Bond novel that reads like a Bond film, then the writer who comes closest is Raymond Benson, whose works are easily readable as Brosnan-era Bonds in book form, particularly in the climax to his Union trilogy, Never Dream of Dying.
If one is looking for a better balance (the characterizations of Fleming, the fireworks of the EON productions, a decent amount of polish), then I can think of none better than John Gardner's first, Licence Renewed--while also giving a good word to the novelizations. Christopher Wood's novelizations of the scripts he co-wrote for The Spy Who Loved Me and Moonraker were, some distasteful or strained bits apart, a pleasant surprise in that respect.
* At the time of this writing, the author had not yet read Anthony Horowitz's Trigger Mortis (released only this September).
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