Do the BSD light novels have to be read in order to make sense or can we just read them out of order? Are they each standalone? Or do they follow one another ?
From what I can see, you can read each by itself and enjoy it (the stories are complete in themselves), but the stories are all related to each other so by reading all of them you can feel the whole universe of character relationships Asagiri has built. There are many ways to make sense of it, depending on your personal preference! These are the few I can think of:
1. Reading it as the author intended
Asagiri published the novel on Dazai’s entrance exam first, and some things in there are not explained until the second novel, then the third. If you want the sense of surprise and revelation and “oh so THIS is what they meant!” that the author intended, reading it in published order would be the best way to go!
(Entrance Exam -> Dark Era -> Untold Story Chapter One -> Untold Story Chapter Two -> Gaiden)
2. Reading it chronologically
If you want to get a sense of how time passes and how the characters have changed, then chronologically is the way to go! You will get the full impact of character development that way.
(Untold Story Chapter Two -> Dark Era -> Entrance Exam -> Untold Story Chapter One -> Gaiden)
3. Reading according to your favourite characters
If you don’t mind not knowing everything in all the novels, picking which to read according to the characters you like is fine too, but you might be missing out on some things in the manga canon (since Asagiri likes to bring novel references into the manga story)!
Dazai: …pretty much every single novel, except Untold Story Chapter Two and GaidenKunikida: Entrance Exam, Untold Story Chapter OneFukuzawa and Ranpo: Untold Story Chapter TwoAkutagawa: A small part of Entrance Exam, Dark EraOdasaku: Dark Era, Untold Story Chapter TwoAngo: Dark Era, GaidenTanizaki: Untold Story Chapter OneMori: Dark EraArmed Detective Agency (except Fukuzawa, Atsushi, Kyouka): Untold Story Chapter OneChuuya: A small part of GaidenAtsushi: Out of luck for now! The fourth light novel will probably change things! (*dances in seat*)
Hope this helps! Enjoy~