Beat 'em up articles

Here's a good 2 parter, written in 2013(it dates itself by mentioning modern/upcoming titles in the 2nd part), that goes over the history of the genre:
http://fextralife.com/a-history-of-the-side-scrolling-beat-em-up-part-1/
http://fextralife.com/a-history-of-the-side-scrolling-beat-em-up-part-2/

Racketboy's pair of articles is pretty good too:
http://www.racketboy.com/retro/beatemups/beatem-ups-101-all-you-need-to-know-about-brawlers
http://www.racketboy.com/retro/the-best-undiscovered-beat-em-ups

There's always this if you want to read about some games/series in more depth:
http://www.hardcoregaming101.net/catalog/genre-bmu.htm
Though some are more platformer with melee combat, or something else entirely. "Kusoge" basically means crappy or awkward.

http://shmuplations.com/ despite how genre centric that sounds, has been translating old interviews with Japanese developers, and sure enough these include beat 'em up games.

http://shmuplations.com/undercovercops/
^Basically goes over the creation of the female character, but at the bottom of the page they have a download link to a translated manga based on the game. Note that said manga might not be suitable for work, or very mature in general.

http://shmuplations.com/capcom1991/
^An overviews of Capcom in 1991, it goes over Final Fight and Street Fighter II at the same time, as well as the Tenchi Wo Kurai series

http://shmuplations.com/rtypesound/
^Even though it's about the sound composition of R-Type, it makes a brief mention of the sound composition of Kung Fu Master.

http://shmuplations.com/dnd/
^1996 interview about Dungeons & Dragons: Shadow over Mystara.

http://shmuplations.com/dnd2/
^2013 interview about Chronicles of Mystara, the digital distribution package of both of Capcom's D&D hack 'n slash beat 'em ups.

http://shmuplations.com/streetfighterii/
^Even though it's about Street Fighter II, the beginning mentions Final Fight and it's history as the intended Street Fighter II.

http://shmuplations.com/kinunishimura/
^Very brief mention of being inspired by Final Fight near the beginning of the article, and very brief mentions of her character design work on Power Gear(Armored Warriors) and Shadow Over Mystara toward the end.

http://shmuplations.com/streetsofrage2/
^Interview with Ayano Koshiro, the Streets or Rage 2 character designer.

That's about it for this genre so far. There are plenty of other types of games with translated interviews over there, so be sure to check those out if you're interested.