I found this hardcover on the charity-sale shelves at a local Hannaford's. It's the 52nd (!) book in the "Magic Treehouse" series, and the fourth in a mini-series of challenges for siblings Jack and Annie. Here, they're out to find the fourth "secret of greatness," having already tagged Alexander the Great (and his horse), Harry Houdini and his wife Bess, and Florence Nightingale. Next up: soccer star Pelé!
The story involves some ok-in-fantasy-but-not-in-real-life bits, such as the kids sneaking out of the house in the middle of the night without notifying their folks, to heed the summons of the Magic Tree House. (This kind of thing wouldn't have attracted my notice when I was a kid, but now I can't help imagining the folks getting up in the wee small hours, peeking in to check on their sleeping kids, and being horrified to find them missing. My mom lit into me once when I woke up unusually early one day and decided to go out for a walk... I recall being mad at *her* for peeking in on my while I was sleeping, so there you go!)
Anyway, once the magical adventure kicks in, the kids - magically able to communicate in Spanish - find themselves in Mexico City in 1970 for the final game of the World Cup. They even have tickets in the best seats in the house - if only they can make their way to the stadium via complex public transit... That part of the story gets tense, a true urban nightmare of being lost in a strange place on crowded trains. But not to worry - a friendly local kid, also bound for the stadium, comes to their aid. In return, Jack offers to swap his deluxe seat for Roberto's nosebleed-section seat, as Roberto is watching the game on behalf of all his siblings... Very sweet gesture by Jack, though things don't quite work out the way he hoped they would.
We only get snippets of the game before our heroes try to make their way home, with a side trip to Roberto's aunt's neighborhood - where they indulge in a lovely game of scratch soccer, barefoot in the muddy streets. (Another "hmmm..." bit here: since none of the three protagonists are very good at soccer, they use Merlin's Magic Mist to make all three of them play like Pelé. I couldn't help thinking that using magic to cheat at a friendly neighborhood game seems wrong, even though it did help earn Roberto some respect. Luckily, the other kids were so impressed by the mad skills that they didn't mind losing; it'd be great if that were the default in sports, celebrating skill regardless of whose team displays it.)
After a very busy and exciting day, our heroes bid farewell to Roberto and head back to the tree house, where they complete their four-secrets mission - and wind up in a kind of "greatest hits" scene in Camelot, with cameos from nearly every character and adventure they've been on to date!
The author's notes are entertaining; it seems she got to meet the daughter and granddaughter of Pelé (though not the man himself), and learned a lot about the game while researching the book - which she undertook due to all the requests from her fans for a soccer-based story. There's also a mini-bio of US soccer star Mia Hamm.
[There's a TV Tropes page on the series, with some entertaining tidbits.]