The plot, as described on the work's article, is really quite simple, and feels more like an excuse to get three unrelated kids - the brain, the tattletale, and the doofus - together and to see them interact with each other. And it works really well, since the character interaction is so fun and feels so natural. This is the case even though the characters are somewhat cartoonishly exaggerated in terms of personality.
Maxie uses little-known words that are actually listed in a glossary - "Maxie's terms" - at the back of the book. Rosie is proud of her sneakiness and ability to tell on everyone without them finding out it's her (most of the time). Earl is so afraid of being caught getting involved in all this that he dresses up in a heavy overcoat and ski mask when he shows up at Maxie's house to discuss strategy. It fits well with Earl's neurosis. He's also paranoid of his mom overhearing his phone call to Maxie and Rosie.
Interestingly, even though Earl is the Butt Monkey
in some ways, he's the one who gets to shine the most. It's his idea to get out the phone book (remember those things?) to try to get ahold of Maxie and Rosie and get together with them. He's also the one who calls up the kindergartner on the phone who may have witnessed their sneaking out of school, and try to get her not to tell. Rosie also gets her own bit of character development, as this situation helps her kick the tattling habit. Maxie's probably the "coolest" of the nerds to begin with, and that doesn't change.
It's a very fun book, entirely because of the characters, and it really makes me wish for more of these characters as a group. Barbara Park did write a few spinoffs, which focus on one character each, but I don't think she could really top the fun mixture of personalities she's got here.