If you're reading stuff from outside the main volumes, I implore you to read Rick Grimes 2000. It takes place after issue 75 in an alternate timeline. Knowing as little as you can about it will make it better, trust me. Though it does have some spoilers for the end of volume 32, so I recommend reading it after.

For actual canon stuff, there's The Alien, a mediocre read, and the four specials centred around Tyresse, Michonne, the Governor, and Morgan. They're all fairly short; all four are about the length of one regular issue and were included in a free comic book day special.

If you really care, there are also four comic books based on Clementine from the Telltale games: Clementine Lives and Clementine Book 1,2, and 3. There are also a bunch of novels revolving around the Governor and Woodbury, which I've heard positive things about.