It's still ongoing! To date there are 6 issues. The comics are mostly framed around tales that Doc tells Clara and their children Jules and Verne about his life and friendship with Marty. This is setup after Marty goes back to the future for the last time in the third film and before Doc comes back to the future too with his family.
#1
When Marty Met Emmet
It's a little fun story about the first meeting of Marty and Doc and how the young Marty became his lab assistant.
Looking for a few good scientists.
It deals around Doc Brown involvement in the Manhattan Project when Doc Brown was a young excentric professor at Caltech.
#2
The Doc Who Never Was
After the events of the first film, Doc Brown finds himself needing a lot of money in order to complete a functional version of the flux capacitor. Old figures from his past involvement in the Manhattan Project come with the request that would give the army a unlikely and powerful weapon that could lead to terrible consequences.
Science Project
A little funny story about Marty trying to make an assignment for school and trying to make sense of some of Doc's other inventions.
#3
In search of Calvin Marty Klein
George McFly and Lorraine relationship is in trouble time after the events of the first film and there's only one person who can help them: Calvin Klein! In his absence, Doc tries to fix the situation with hilarious and endearing results.
Jurassic Biff
Mean Old Biff Tannen steals the Delorean in the future to travel to the past in order to give important information to his younger self... only he doesn't know how to operate the time machine and winds up farther than expected...
#4
Peer Pressure
Marty McFly meets Jennifer and tries to be with her, only to be mocked by Needles in his truck. Feeling embarrassed by his lack of wheels, Marty becomes tempted to use the secret vehicle Doc has been working on.
Emmet Brown visits the future
We find out how it went the first trip of Doc Brown into the far away future of 2015! Also how he managed to get money in order to make the Delorean a flying vehicle.
#5
Clara's Story
This is a very personal and heartwarming tale about the life of Clara Clayton, her hopes and dreams.
And I haven't yet read #6, but it is building into something awesome!