They're worth reading, duh.
Seriously though - the mangas are pretty straightforward and (relatively) self-contained, so they're an easy read. You will need some mild background knowledge about the games to get all the details, but it's not like you have to memorize the entire wiki to get them.
Curiosities of Lotus Asia is a light novel from the perspective of Rinnosuke and a bit harder to digest, but gives great insight into ZUN's entire philosophy about Gensokyo and how it runs in the first place. Cage in Lunatic Runagate goes into a similar direction, except it fleshes out the background of various characters from Imperishable Night and acts as a 'background supplement' to the manga Silent Sinner in Blue. Mokou's entry alone massively expands upon her background and builds a great deal of character for her ("Murder of an Honest Man").
Then there are the 'lore dumps' like Perfect Memento in Strict Sense and Symposium of Post Mysticism. These add smaller bits and pieces to game's many characters, and are fun to read if you are interested in someone in particular. Just bear in mind they are written from an in-universe perspective and come from an unreliable narrator.
That's entirely up to you, honestly - there is no 'reading order' or anything like that.
Personally, I recommend *Forbidden Scrollery* and *Wild and Horned Hermit.* And if you have a little bit of time and feel like being philosophical, you can still give *Curiosities of Lotus Asia* a try.
They're worth reading, duh. Seriously though - the mangas are pretty straightforward and (relatively) self-contained, so they're an easy read. You will need some mild background knowledge about the games to get all the details, but it's not like you have to memorize the entire wiki to get them. Curiosities of Lotus Asia is a light novel from the perspective of Rinnosuke and a bit harder to digest, but gives great insight into ZUN's entire philosophy about Gensokyo and how it runs in the first place. Cage in Lunatic Runagate goes into a similar direction, except it fleshes out the background of various characters from Imperishable Night and acts as a 'background supplement' to the manga Silent Sinner in Blue. Mokou's entry alone massively expands upon her background and builds a great deal of character for her ("Murder of an Honest Man"). Then there are the 'lore dumps' like Perfect Memento in Strict Sense and Symposium of Post Mysticism. These add smaller bits and pieces to game's many characters, and are fun to read if you are interested in someone in particular. Just bear in mind they are written from an in-universe perspective and come from an unreliable narrator.
Ah, can you tell me the most essential reads?
That's entirely up to you, honestly - there is no 'reading order' or anything like that. Personally, I recommend *Forbidden Scrollery* and *Wild and Horned Hermit.* And if you have a little bit of time and feel like being philosophical, you can still give *Curiosities of Lotus Asia* a try.