Ian Fleming's output as a novelist can be broken into three periods. The first period spans Casino Royale and ends with From Russia With Love. I don't know where the second period ends. It might be with Thunderball, or two novels down the line, with On Her Majesty's Secret Service. It's hard for me to tell, but the consensus seems to be that the final period is marked by Fleming's failing health, his dissolution with the Bond character/format, and his waning interest toward continuing the series. Thunderball is the first novel that breaks with the old Bond format, in that it began not as a novel but as a screenplay. This fact would come back to haunt Fleming (and later the company which produced the Bond films) for decades. Thunderball was originally meant to be a film, and of all the novels it is possibly the most cinematic in style and scope.
Thunderball began as a collaboration between Fleming and several other men, one of whom is/was an opportunistic chump called Kevin McClory. The screenplay yielded some ideas, but was eventually abandoned. Unfortunately, Fleming drew from the abandoned screenplay to produce the novel Thunderball. He was promptly sued by the surviving collaborators (or, at least McClory), who claimed they/he co-owned the source material, everything to do with Thunderball, and therefore also James Bond. The case was eventually settled, and when the filmmakers opted to adapt the novel to the screen, McClory wrangled an Executive Producer credit. For the next 15+ years, McClory would wrestle with these his fragments of ownership, attempting repeatedly to make his own Bond film(s). All McClory's scheming was derailed when courts ruled that the only Bond-related material McClory had any claim to was the story for Thunderball. In 1983 he took that right and, with an enriched Sean Connery in tow, produced/released the uneasy Never Say Never Again. But let's nevermind about the films for now.
For me (at least during this most recent re-reading), Thunderball is the third of Fleming's great Bond novels in the series (the other two being Casino Royale and From Russia With Love). I don't mean that Thunderball places third for quality ranking (though it probably does, actually), but that it was the third time he'd written a great Bond book, rather than just a fair or good one. Whether it has to do with the richness of the collaborative source material that inspired it, or if he was simply in rejuvenated form, for some reason the book seems more evenly gripping.Â
Fleming's own health was beginning to deteriorate after a lifetime of excessive living and vice. Thunderball opens with Bond feeling wretched, diagnosed as being in a biologically toxic condition, and on the receiving end of one of M's temperamental fads. He sends Bond to a health spa called Shrublands, where 007 will be decontaminated, cleansed, and healthified. Bond's near hysteria at the absurdity of M's decree is probably the funniest prose Fleming has ever written, and Shrublands itself is filled with head-shaking quackery and other amusing jazz. Bond eventually graduates from Shrublands, healthy and exhibiting hippy-dippy behavior that has his peers doubting his sanity. After a near death experience, Bond snaps out of his recent wellness, and has a righteous ration of goodly bacon and scrambled eggs, chased with proper drink and cigarettes, before departing on his mission.
The novel marks the debut of the criminal organization called SPECTRE, and Thunderball shows SPECTRE at their most plausible and straightforward: we have stolen two atomic bombs, and we are holding the western world ransom for a vast amount of money. The puzzling thing, which has never been clear to me, is why Blofeld, the leader of SPECTRE, is called "Number 2" in Thunderball, and Largo, Blofeld's subordinate representative, is called "Number 1". Largo is not superior to Blofeld. It's just one of those things that probably should have been corrected during revisions and was never adjusted. Blofeld is, likewise, at his most plausible here. A minor character when considering the actual page-time that he occupies, Blofeld lacks the rather loopy hubris that undermines him in later novels. Here he's just a criminal genius, rather than a vain supervillain.
Domino, the "Bond girl" of the novel, manages to be very sympathetic for some reason. Largo's kept-woman, she seems like a lost twenty-something who recently fell in with a bad crowd, and is now in over her head - she just doesn't know it yet. In the novel, it's as though she hasn't known Largo more than a few months, whereas in the film I always got the impression that she had been Largo's mistress for at least a few years. The Domino of the film is attractive, but rather aloof and bratty. She seems like a rather bored socialite. In the novel she is more multifaceted and sympathetic. Her imaginary love-affair with the sailor depicted on packets of Player's cigarettes is one of the best examples of Fleming writing emotions well.
As much as I love the character of Felix Leiter, in Thunderball he begins to feel a bit contrived and out of his league. The man has several artificial limbs, yet he insists he be allowed to participate in an underwater knife battle. It just seems forced. Like I said, I love the character, I wish he wasn't always being hoisted on Bond whenever the action takes place near American territory.
Thunderball is a really good read, and I recommend it without reservation. Of course I'd recommend anyone start at the beginning of the series and work along in that sequence, but if you're someone who loves the films and wants to start on the books gradually without being too far removed from the familiar, Thunderball is about as close as you'll get from Fleming.