Ah, you started at the first book, which is arguably one of the most boring of the lot for an adult in the 2020s. I suppose in 1954 the idea of giant nuclear-powered VTOL aircraft was pretty awesome, but it doesn't have the same wow factor today. The rather bland obstacles and enemies don't help any either, although that's rarely much of a factor in the Swift Jr. series - he doesn't even a recurring peer rival like (say) the Three Investigators did. I can also confirm that there is no angst or internal dialog anywhere in the series, which I can do without in this kind of kid lit, frankly.
I do question whether the series was aimed at readers anywhere near as old as twelve, much less the bulk of the YA bracket. I started on them in the summer between 1st and 2nd grade and even at that point I knew they kids' books compared to the Barsoom stuff I was also getting from a different relative. Aside from some the odd bit of semi-accurate science and invention names like "megascope space prober" (my own introduction to the series, and another rather dull invention even then - it's a fancy equivalent to a 1960s era telescope) there's nothing in those books to strain anyone reading past kindergarten comprehension levels, and I was sharing books with interested classmates while our ages were still in the single digits. Can't imagine anyone in high school feeling comfortable reading these in public back in the Sixties and early Seventies.
That said, if you ever do take another stab at them despite lacking a nostalgia drive to help, I'd rec something with a more far-out central invention since that's usually the core appeal of the book. Straining my memory a bit, good choices might be:
Outpost In Space (1955) - The idea of building a space station out of multiple rocket ships isn't very exotic today, but it's an interesting piece of history to see how 1955 thought it could be done. Bit like watching those old Disney educational films about space flight and life on Mars.
Race To the Moon (1958) - Aside from more Cold War space race vibes, this one introduces the Challenger, a signature spacecraft that will largely replace conventional rocket-driven craft from here on and runs on "repelatron" drives that are another bit of handwavy tech that get used in many other books. This marks something of a turning point from somewhat serious speculative fiction to increasingly wild scifi. Interesting that it only took till 1958 for rockets to be kind of old news.
Deep-Sea Hydrodome (1958) and Spectromarine Selector (1960) - If you're going to read one of these, you might as well read the other. The former is largely about developing tech for resource extraction on the sea floor using (get used to this) repelatron tech to make air-filled domes for people to work in comfortably. As usual there's some folks who want to steal and/or destroy the tech for themselves, and (for Tom) quite a few teething problems in getting it work in the first place. The second book takes that hydrodome concept and uses it for underwater archeology, but first Tom has to invent a gadget for cleaning up the ruins which are covered in toxic slime and seaweed.
Repelatron Skyway (1963) - Speaking of both gonzo inventions and repelatrons, how about building a floating highway bridging an impassable area of swamp and jungle to help a developing African nation? And that despite having invented flying cars/trucks back in Triphibian Atomicar (there's an excuse for not using them, however thin). The representation of the local populace is the biggest sticking point in this one, but no more intolerable than average for the early Sixties. Nice reminder that Swift Enterprises started out as a the Swift Construction Company in Tom Sr.'s day and does, in fact, have experience building roadways.
Subocean Geotron (1966) - I may be biased because this one shares my birth year, but this a particular favorite. The titular geotron is a submersible tunneling machine meant to prospect for sea-floor mineral deposits. It winds up being vital for recovering a bunch of alien zoological specimens sent to Tom by his "space friends" on Mars (maybe) that go astray and wind up in the briny deep. The final form of the geotron is crazy looking (and uses repelatron tech again - shocking) and the alien flora and fauna are well described and pretty nifty - at least for a nine year old me.
Captive Planetoid (1967) - What do you do when an asteroid made of sapphire is on a collision course with Earth? Boring pragmatic types would try to deflect its course using, say, repelatrons. Tom sticks a giant parasail glider wing on it and arranges to soft-land it in a mountain of foam instead. Add some very ambitious jewel thieves to the mix and it's a fine bit of silliness.
Those are probably the highlights, although YMMV and they'll never be more than what they are - roughly seventy year old kids books that helped inspire quite a lot of future scifi writers and actual scientists.
