Personally, I loved the I SPY novels (which is why I adopted Walter Wager's pseudonym as my forum moniker); and even though they were grander in scope, they seemed to me perfectly apt -- at least insofar as they represented an authentic view of the characters as Walter saw them, and as he could absorb them into his own imprimatur. Tie-in books these days are much more the product of devoted fans (even if they're pro writers working on commission), and syncing up with official canon and timeline is pretty much a mandate ... which is not remotely a bad thing. But in the 60s, tie-in novelists tended to be seasoned pulpsmiths or genre writers and their literary versions of popular TV shows were very often (albeit informally) "alternate universe" takes, with their own discrete lore. The variations could be offensive or just plain clueless (i.e. Michael Avallone writing HAWAII FIVE-O's Steve McGarrett as a standard paperback ladies' man); sometimes they could be thoughtful (in a circumstance that would be rejected out of hand these days, Keith Laumer agreed to write the first two INVADERS novels ONLY if he could change some things about the premise, which is how David Vincent was suddenly an aerospace engineer rather than an architect); but they brought with them very strong, muscular and distinct literary styles (if you can locate it, Jim Thompson's take on IRONSIDE is as memorable as any of his noir thrillers.) I read Walter's I SPYs as an impressionable pre-teen, at first, and they had a huge influence on me as a budding writer. One I still feel.
The 60s were also the days before VCRs, so a tie-in writer only had scripts, photos and (if he was lucky) the episodes he watched on the tube at the same time as everybody else. (Even today, a writer might not actually SEE the series he's writing about, if the merchandising machine wants the books in the store early. TORCHWOOD, the spinoff of the the new millennium DOCTOR WHO, just spawned three original novels [BBC Books] that came roaring onto the shelves right after the last of the first season's 13 episodes aired. The novels are excellent pastiches, even better than the episodes themselves, I thought -- and not one of the three authors, Peter Anghelides, Dan Abnett, nor Andy Lane, had seen an episode until after they were done.) As to the I SPYs, I think the first book was Walter's learning curve, but after that he was flying. And I looked forward to his I SPY universe as much as the one on TV. I thought the creation of their boss, Donald Mars, and the particular hierarchy of teams they were a part of, were inspired. I dug it that they had a team identity, that Mars had dubbed them "Domino." The code names were a blast, as were the larger-than-life (yet credible) villains.
And despite Mr. Cushman's assertion that Kelly and Scott worked for Military Intelligence, I thought it made perfect sense for Walter to assume they were CIA. Indeed, I'm not so sure Cushman's right and Walter's wrong. If you actually look up what Military Intelligence is, it's not a division or a department, it's a specialized kind of information and information gathering and it really does pertain primarily to military matters -- not the kind of thing Kelly and Scott would be involved in. Indeed, most of their cases were classic CIA-type assignments. I think the series never NAMED their agency (you'll remember that in time their agency was vaguely referred to as "the department") precisely so that the writers could keep it loose and give Kelly and Scott the widest possible lattitude and jurisdiction (i.e. if your cover is as an internationally known tennis bum, how can you expect to fit convincoingly into a small town for "Trial By Treehouse"? And what in the world does that episode have to do with anything military? Or anything CIA, for that matter, as "in country" ops would be the province of the FBI?); and I think the military fellow in the early episode who is said to have teamed them initially (and any such else that came in its wake) was a convenient invention for the episode and an early continuity sacrifice. Kelly and Scott were simply not military personnel. I would even go as far as to say that by creating Donald Mars, Walter Wager was unintentionally anticipating the creation of Russ Conway -- who is clearly the kind of black ops "suit" who would be a veteran CIA man.