The door in the alley / Adrienne Kress ; illustrated by Matthew C. Rockefeller.

Enhanced descriptions from Syndetics:

Featuring a mysterious society, a secretive past, and a pig in a teeny hat, The Explorers: The Door in the Alley is the first book in a new series for fans of The Name of This Book Is a Secret and The Mysterious Benedict Society. Knock once if you can find it--but only members are allowed inside.

This is one of those stories that start with a pig in a teeny hat. It's not the one you're thinking about. (This story is way better than that one.)
This pig-in-a-teeny-hat story starts when a very uninquisitive boy stumbles upon a very mysterious society. After that, there is danger and adventure; there are missing persons, hired thugs, a hidden box, a lost map, and famous explorers; and there is a girl looking for help that only uninquisitive boys can offer.
The Explorers: The Door in the Alley is the first book in a series that is sure to hit young readers right in the funny bone.

" [A] wildly funny adventure ....Animals in teeny hats, Wonderland-style logic, and loads of wordplay and sarcasm will keep readers giggling all the way through. "- Kirkus Reviews

" Exhilarating....Fans of a Series of Unfortunate Events will be drawn to this. "-- Booklist

" A rollicking read, full of derring-do and old-fashioned villainy. "-- School Library Journal

" Funny, offbeat, and subversive ...occasional footnotes and other humorous asides from the omniscient narrator break the fourth wall and ramp up the playfulness." --The Horn Book

"Narrated with a smart, brisk tone and plenty of snark... The Door in the Alley packs plenty of twists, turns and danger. "-- Shelf Awareness

" The Explorers: The Door in the Alley is a rollicking and clever adventure! Chock full of brilliant plot twists, pitch-perfect humor, and non-stop action. Kress has kicked off this series with a bang!"--Arthur Slade, author of The Hunchback Assignments