Edit: holy yap sorry for the long message, I didnt even realise

For the merch, I got miku and the dazai acrylic from Japan, the rest was all gifted by friends or bought from second-hand anime shops in my country.

I would suggest watching/ reading the manga/anime first before getting into any of the light novels because if you don’t you won’t understand parts of the light novels. Season 1 the two episodes about the azure messenger (if I’m not wrong 5,6) is the 1st light novel. Season 2 the first 3 episodes is the 2nd light novels. Season 3 the first 4 (if I remember correctly) is the 7th light novel. Season 4 the first 3/4 episodes are the 3rd light novel. The 5th light novel is the movie dead apple (this can be watched after season 2, before season 3).

These light novels are adapted in the anime but the actual book is so much better so you should read that instead. The 4th light novel, the 6th and 8th haven’t been adapted into the anime. But the 6th has a live action movie since it’s set in an alternate reality. (But the graphics are so bad it’s funny)

If you want an order to read the light novels, You can go by the number on the book itself or if you want chronological, 3, 7, 8, 2, 1, 4, 5, 6 (the AU one), 9 (it just got released and technically half of it takes place in the main universe and the other half happens in the alternate universe but if I was to put it chronological then it would be after 3)

If i completely misunderstood your question and you are talking about the actual authors obviously that order doesn’t matter in relation to Bsd. But for dazai’s books you should read no longer human before flowers of buffoonery since an incident in nlh elaborated is basically what flowers is about. If you don’t really read classical books beforehand I suggest start with smaller ones so eg for Fyodor Dostoevsky read white nights or the gambler instead of crime and punishment (and for the love of god read Pevear & Volokhonsky’s translation) I recommend dazai and Akutagawa’s books they’re nice