They all meet when they're initially struggling to get their books published, and so all of them pool their savings (and a significant donation from from Combeferre's mothers) into self publishing.
Eventually their books prove to be popular enough for them to have a proper little printing company of their own- it's not huge but they all love it anyway.
Enjolras writes those illustrated social justice books to teach children about human rights, and writes simplified biographies of famous figures in various political movements. He also puts similar messages into more traditional stories to try and make sure that parents prejudices are or passed on to children as easily.
Courfeyrac tends to write joke books full of terrible puns, but he's also loves pop up books. He's published the least of any of the amis, but he has also read every single book any of the others have written.
Combeferre writes those 101 fact books about basically every subject that interests himself. He's written dozens of books on science and space, they're his favorites.
Joly writes books of fun home science experiments to get kids interested in chemistry. He tests them all out at home, and on the weekends him, Bossuet and Chetta will all make a complete mess of their house with baking soda volcanos (and have a heap of fun doing it).
Gav is his official tester, he gets the first copy of every new book as soon as Joly has it and he won't put anything in the book with his seal of approval.
Combeferre and Joly are also working on a collection of books about the human body. So far they've written three.
Jehan writes poetry books, which have notes and encouragement for kids to try and write their own. They've also done books of those fun shape poems, again with helpful hints about writing and forming your own, and with blank shapes left at the back for kids to fill in their own.
Bahorel does traditional moral stories with a modern twist. He wants his books to teach children to be kinder to one another, and to other people. Him and Enjolras have been working together on trying to write books about racism without being too overt.
Cosette writes books about hobby crafts and other fun activities for children. She began publishing with ABC because she was being pressured to publish her books as those 'crafts for girls' things and she didn't believe in gendering them so heavily. She helps Musichetta with her children's cook books too. Typically she writes the baking sections because she can include recipes that baker!Valjean did with her when she was a child.
Eponine writes those versions of fairytales which are messed up or told from the villains point of view or whatever. She could never get them published because they were kind of dark.
Marius does traditional fairy tales illustrated really well (with princess who tend to look increasingly like Cosette) and is the only one of ABC and friends to ever have something published by a major publisher.
Grantaire and Feuilly are both illustrators. They both have very different styles- Feuilly typically illustrates Enjolras' books since his style is more traditional, where Grantaire's more cartoonish style fits better with Bahorel and Joly's styles.
Feuilly has also co written a book with Cosette about origami.
Grantaire could always see how much joy his friends got from creating stories and decided to try it himself. He finds inspiration in the people that he loves, rewriting traditional fairytales with them as characters.
It actually started with him writing "The democratically elected leader and the frog" as a joke, but people actually love them? And eventually everyone has their own.
Montparnasse doesn't talk to R for a week after the publication of "The (Gender Fluid) Beauty and the Beast" because of the rather unflattering caricature of him.
His most popular three are still his presentation of the ABC and friends in Robin Hood, his reimagining of the little mermaid (wherein rather than melting into sea foam princess Chetta marries the prince and his husband) and his lesbian version of Rapunzel (featuring eposette).