There is no order of reading, just read the one which seem the more interesting to you. As for the relation with Sekaiitchi Hatsukoi, they are from the same author and sometime the characters of one story can be seen in the background of the other story. Both of them have an anime by the way.
Thank you! I was kinda worried where to start with all the Junjou franchises? Egoist/Terrorist/Mistake! and hnnnggh I actually just started watching JR, how come I missed something like this (TT)
Ah, if you're confused, let me explain:
Junjou Romantica delves into the lives of four different couples, and is mainly focused on the shenanigans of the Romantica couple, involving Takahashi Misaki (a college student attending 'M' university) and Usami Akihiko (a popular novelist).
The other couples are viewed as side pairings, therefore, their stories can be read separately, but it'd be better for you to go according to the order that has been posted here. So, the other pairings include:
Egoist: Kamijou Hiroki (a literature professor teaching at 'M' university, who is also Akihiko's childhood friend) X Kusama Nowaki (an orphan who aspires to become a pediatrician).
Terrorist: Yo Miyagi (a divorced literature professor at 'M' university, who is Hiroki's senior) X Takatsuki Shinobu (a college student, his father happens to be the dean at 'M' university).
Mistake: Ryuuichirou Isaka (a senior director at Marukawa Publishing - linked to the Sekaiichi Hatsukoi universe) X Kaoru Asahina (Isaka's secretary and childhood friend).
So, to an extent, everyone is linked to each other in an interesting way. Akihiko often publishes his novels with Marukawa Publishing, so he's well acquainted with the Mistake pairing, & etc.
Often, you may see the characters from Sekaiichi Hatsukoi appear here, because it's been established that the events ensuing in both of these mangas occur within the same timeline. If you read the manga from the very beginning, you'll see a drastic improvement in the art style, and... let's just say that various issues become less problematic with the progression of the series.
I hope you have fun reading ╮(￣▽￣)╭
JR came first and SH was a 'spin off', I believe. The characters in SH work at the publishing company that Usagi writes for (Emerald). Every once in a while one will have another character from the other manga in the background. But generally they're separate. If I remember correctly Isaka is the one who discovered Usagi as a writer, and Isaka is also Ritsu and Takano's boss.
I believe that's the main link between them. But other fans, please do feel free to correct me if I have it wrong.
Can any kind soul direct me which to read first? and how does this relate to sakaiichi hatsukoi?