What I personally find interesting isn't the book itself, although it's definitely an oddity in and of itself in Wodehouse's writings, but the fact that I was not only read it for free but at all.
When I first got into Wodehouse, which was close to twenty years ago now that I think about it, the Swoop was one of his most obscure stories. It didn't even get printed here in the US until after Wodehouse's death AND it had been written in 1909! I saw it once as a battered hardback in one library.
And now, thanks to the power of public domain and Project Gutenberg, I was able just download it to my phone and read it for free in about a half hour. Seriously.
I love living in the future. Our day to day life is the science fiction of my childhood. It. Is. Awesome.
The Swoop is an oddity in Wodehouse's work. While just about everything the man wrote was a comedy or at least really funny, the Swoop might be the only actual parody that he wrote. More than that, it's a parody of Invasion Literature, which is not the genre it was back at the start of the twentieth century.
Fortunately, I had read the Riddle of the Sands by Erskine Childers so I had at least some idea of what Invasion Literature was all about. Back in the day, it was all about how England wasn't prepared for war and invasion and what steps needed to be taken to safeguard her shores.
In the Swoop, nine different armies invade England on the exact same same day. England, while annoyed at how this disrupts the Cricket season, treats them like tourists and overcharges them for goods and services. In the end, the Boy Scouts drive them out and save England.
While the Swoop doesn't have the sparkle or brilliance of Wodehouse's later, mature books, it is still pretty funny. Some of his other early work, like Love Among the Chickens, are actually dreadful but the Swoop is silly fun.
The Swoop isn't what I would recommend for someone wanting to explore Wodehouse through the public domain. There are much better works available there, like Mike and Psmith or The Clicking of Cuthbert or My Man Jeeves. But the Swoop was still an amusing discovery.