
The two most-asked-for series at my library are definitely the
Diary of a Wimpy Kid* series, by Jeff Kinney, and the
Captain Underpants series, by Dav Pilkey. Unfortunately, the probability of a book in either of these series actually being in the library is about the same as the probability of your vacuum cleaner coming to life and chasing you around the living room.
But never fear, I am prepared**! Whenever someone wants some Capt. U and I can't find any on the shelves, I recommend Franny K. Stein, which is about an elementary school-age mad scientist. It has robots! And lots of zany pictures! And it's written at the same, streamlined reading level.
And whenever someone wants Diary of a Wimpy Kid, I recommend My Dear Dumb Diary. Once again, it has it all: cartoony diagrams, short entries about middle school life, lots of exaggeration, and a slim, unintimidating spine. It's just what you want when you have a book report due tomorrow.

Amazingly, both of these series, which obviously tap into some elementary school zeitgeist, are by the same author:
Jim Benton. Just listen to the titles of some of his books: "The Problem with Here is it's Where I'm From," "Never Underestimate Your Dumbness," and "My Pants are Haunted." If I was in charge of giving a literary award, I would give it to him. Meanwhile, I will recommend his books to everyone I see. So watch out.
*BTW, Diary of a Wimpy Kid was originally published as an online novel or webcomic or waddayacallit, and you can still read Greg Heffley's Journal on
http://www.funbrain.com.** Not for vacuum attacks.