Where to Start: Middle-Grade

So you've discovered middle-grade fiction. It's great isn't it? I personally think some of the most creative books are coming out of middle-grade right now. If you're not reading middle-grade, you're seriously missing out on some fabulous books.
The definition of middle-grade can be...a bit broad. But for this list, I categorize it as books that deal with characters from ages 10 to 13/14ish. Mostly because that's usually how publishers categorize it. If you're ever unsure about whether or not a book is YA, Childrens, or Middle-Grade I suggest going to your local Barnes and Nobel and seeing where it's shelved. B&N tends to put books where the publishers tell them to put the books, and what it's being sold as, is usually what it is. FYI, the middle-grade section in these stores is in the childrens area and put under ages 9-12.
As with the classics, my advice is to start with MG books that happen to fall under genres you already love. There are also some staples out there which are a good idea to just read if you want to hold any sort of conversation with middle-grade book fans.

Staples you should read pronto:
Harry Potter series by J. K. Rowling (yes, this is technically MG, though I say it's both MG and YA)
Percy Jackson series (and spin-offs if you want) by Rick Riordan (same)
A Series of Unfortunate Events series by Lemony Snicket (especially since there's going to be a Netflix series starting next year)
Artemis Fowl series by Eoin Colfer

General Middle-Grade:
Holes by Louis Sachar
Maniac Magee by Jerry Spinelli
Because of Winn-Dixie by Kate DiCamillo

If you like Historical Fiction:
Number the Stars by Lois Lowry
The Witch of Blackbird Pond Elizabeth George Speare
Anne of Green Gables series by L. M. Montgomary

If you like Fantasy:
Inkheart Trilogy by Cornelia Funke
Peter and the Starcatchers series by Dave Barry and Ridley Pearson
The Dark is Rising Sequence by Susan Cooper

If you like Sci-Fi:
Animorphs series by K. A. Applegate

If you like Mystery:
The 39 Clues series by various authors
Nancy Drew series by Carolyn Keene (also check out the Hardy Boys series)
The Mysterious Benedict Society series by Trenton Lee Stewart

If you like Contempoary/Drama fiction:
From the Mixed Up Files of Mrs. Basil E. Frankweiler by E. L. Konigsburg
Are you There God? It's Me, Margaret by Judy Blume (also check out Freckle Juice and Just As Long as We're Together)
Diary of a Wimpy Kid series by Jeff Kinney

Personal favs: The Sister's Grimm series by Michael Buckley
Chronicles of Chrestomanci series by Diana Wynne Jones
The Graveyard Book by Neil Gaiman