Double-posting, because I just finished reading the first book from the list and thought it might be useful for some.
Book Title: 怪人二十面相 (The Fiend with Twenty Faces)
Author: 江戸川乱歩 (Edogawa Ranpo)
Difficulty: N4 grammar (sometimes a little into N3, but not much) + very basic, often kana-only vocab
Synopsis:
Detective novel, first of the Boys Detective Club series. It introduces us to Kobayashi, the chief of the detective club, a couple of other club members and the titular Fiend with Twenty Faces. The main story is about the theft of a precious painting collection.
Pros:
- Good to give a rough idea how a book narrative looks like in Japanese
- Overall, relative fun to read
Cons:
- Horrible abuse of kana, making it almost unreadable at times
- Halfway through the book even less kanji is used
- Casually introduces elements specific to the Boys Detective Club, like their BD badge without explaining what these are for
Recommended?
No, sadly. As a first read it’s extremely tough and more frustrating than fun. It feels like the book would suit a native speaker better, one which will for sure be more familiar with the vocab used, even if they don’t know enough kanji at a young age yet.
Book Title: 眠い町 (Sleepy Town)
Author: 小川未明 (Ogawa Mimei)
Difficulty: N4+ grammar (lots of dated expressions and grammar structures)
Synopsis:
The story follows a boy called ケー (not his actual name) who likes traveling and eventually decides to go to the titular “sleepy town” to check out why whoever goes there inexplicably feels tired and falls asleep. What he finds there will shock both him and the readers 
.
Pros:
- very good balance between kanji and kana
- lots of new vocab items, especially verbs
- pleasant writing style
Cons:
- tons of dated expressions, terms and especially spelling
- possibly not suitable as an early read due to the above
Recommended?
Actually, yes. The writing style is very descriptive and one feels like visiting the “sleepy town” themselves. Despite the dated language, the story is fairly easy to follow, enough to get the gist of it.
Covered by one of our fabulous book reading clubs: Week 3: 小川未明童話集 - Ogawa Mimei’s Collection of Children’s Stories
Book Title: 赤いカブトムシ (The Red Beetle)
Author: 江戸川乱歩 (Edogawa Ranpo)
Difficulty: N4+ grammar + some minor N3 points here and there
Synopsis:
A group of 3 teenage girls witness the appearance of a strange man clad entirely in black with a black goatee, looking like a bat. They follow him to an abandoned house in the woods where they witness a possible crime. Kobayashi Yoshio and Kimura Toshio go to investigate. What they find in the house will terrify them to the bone…
Another entry in the Boys Detective Club series.
Pros :
- great plot - really feels like an adventure story
- fairly modern vocabulary, including loads of verbs and onomatopoeia
- better sentence structure than in 怪人二十面相 so it’s easier to decipher snakes of kana
Cons:
- still way too much kana. Later in the book when descriptions with lots of particles kick in, that’s difficult to follow
- probably not ideal if one wants to practice reading kanji
Recommended?
Actually, yes. If one has good kana recognition and some vocab under their belt, reading it isn’t really a chore and the plot is fun as hell.
Chapter 1 of the Red Beetle as audiobook: 【朗読】江戸川乱歩『赤いカブトムシ』第一回【青空文庫】 - YouTube
Book Title: 天空の魔人 (Monster in the Sky)
Author: 江戸川乱歩 (Edogawa Ranpo)
Difficulty: N3 + several N2 points (kanji around N4-N3 level)
Synopsis:
小林君, leader of the Boys Detective Club and two of the top club members, ノロちゃん and 井上君 go for a short trip to the 矢倉 hot springs (not an actual place) in 長野県. There they hear a strange rumor of a monster hand from the sky which cripples cattle and levels fields. A little doubtful at first, they’re intrigued as the crimes of the mysterious hand quickly escalate…
Another entry in the Boys Detective Club series.
Pros :
- very good plot
- fairly modern, well-balanced vocabulary
- some N3 and N2 grammar in context
- more kanji than in the later entries in this series
- doesn’t require being familiar with the Boys Detective Club
Cons:
- weird balance between kana and kanji. After big chunks of sentences with kanji there are suddenly a lot of sentences with almost only kana.
Recommended?
Very much so!
Thank you!