I haven’t really looked over the linked-to Ranpo works (since seeing the kanji numbers scared me off for now), but checking a few random kanji:
彳 appears twice (different stories), and has furigana both times.
I see furigana for 起臥 and 仰臥 across many apperances, but only on first appearance within a story (which I imagine is typically the case).
閂 and 閃 also get furigana on their first appearances.
It’s the third part of a rather long series - maybe you would like try the first, 探偵ガリレオ if you liked suspect x. It suffers from the unfortunate affliction of being ebook less.
Oh, no ebook is a big problem for me. But if it’s a long series, maybe other books in the series do come in ebook form? I’ll have to check.
Some mystery things I’ve read and enjoyed: 硝子のハンマー (start of a series of locked-room mysteries–this particular one is very long, but several of the others are short stories), 孤島の鬼 (Edogawa Ranpo, mentioned above), 十角館の殺人 (an homage to Agatha Christie’s And Then There Were None), 探偵ガリレオ (also mentioned above), and Wの悲劇. Of all of these, most tackleable for a new reader would probably be 探偵ガリレオ (straightforward prose, no outdated kanji/grammar, and it’s short stories). Also, to add a manga recommendation: ミステリと言う勿れ! I find this one thoroughly delightful, and I believe it has full furigana. It’s not an easy one to read, but not too awfully difficult, either. (Often wordy, and sometimes the mysteries involve language and/or kanji.) Lots of interesting mysteries and good characters. 
Thanks for this thread. I have forgotten when I was in secondary school I read almost all of the 三毛猫ホームズシリーズ by 赤川次郎 but of course in translated version. When I Japanese becomes better, I will give the original a go.
BTW, did you mention what the book you just finished was?
三毛猫? A cat is always a good addition to any book 
Are these children’s books though? I hadn’t heard of them.
The book I’m about to finish is コーヒーが冷めないうちに (Before the coffee gets cold), read along with the Intermediate Book Club here. Not a mystery book by any stretch, but quite beginner-friendly in terms of language.
Not chidren’s books. Apparently the cat in the book was based on the real life cat the writer had. I like it so much that I stopped reading other Japanese crime fiction for a long time because no other writers can compared to him until I read 容疑者Xの献身 (translated version).
I see. I flipped through コーヒーが冷めないうちに (translated version) in a bookshop and it was my cup of ‘coffee’.
Thanks for this list. 探偵ガリレオ doesn’t seem to be on Bookwalker unfortunately, but I’m adding the rest to my list to go through later. The manga looks quite interesting too - I’ll keep it in mind along with Case Closed for when I’m in the mood for manga.
This sounds more and more interesting by the minute. I’ll be sure to check out a sample, thanks.
I found a couple of these used outside a bookstore a few weeks back, and I was this close to buying them. (It’s pretty rare to find cheap, used, Japanese books here), but then I remembered my backlog. Is this you recommending them? How old were you when you read them?
That indeed sounds like a recommendation!
Would you say the stories can be read independently? There were a couple volumes, but no 1st volume as far as I could judge just browsing.
Hmmm… I must have been 15-17 there were so many books by 赤川次郎 I just kept reading them. Yes I think you can read them independently.
I have a rule if I see something I like in a second hand shop I will grab it particularly books. 
I looked up Jiro Akagawa (the author), and my, is he prolific! Only in this series there seem to be 38 novels (not counting short stories and such), and according to Wikipedia he’s written more than 560 novels overall! 
I read a little of the sample of the first book, and yes, it’s definitely not for children. Yet it seems rather lighthearted and humorous in tone (at least I think so, I’m still on the very first pages), and the language is straightforward enough so far.
I’m glad you like it 
. What level would you say his books is? Passed N3?
Um, I can’t really answer that I’m afraid, as my approach to grammar is rather random - I just read native content and look up whatever looks like unknown grammar. From what I’ve seen so far - and bear in mind, it’s only a few pages - the language is straightforward, nothing flowery or old-fashioned, no overly complicated descriptions or weird grammar constructs. There is no furigana (with few exceptions), in case that matters to you. It looks about the level of difficulty of Before the coffee gets cold, which is very approachable.
I couldn’t read a sample on Bookwalker because it’s on an all-you-can-read offer (I’m sure there must be a way that I’m missing), but there is one on Booklive if you want to give it a try.
Edit to add: For what it’s worth, it’s been classified as Level 38 (Intermediate, ~JLPT N2) by one person on Natively. By comparison, コーヒーが冷めないうちに is graded as Level 30 (Intermediate, ~JLPT N2). These are all all subjective comparative ratings by readers as far as I know.
Edit 2: I looked up all the books mentioned here on Natively. Some weren’t listed, but the other ones were all in the 30s, with only 硝子のハンマー being higher than 三毛猫ホームズシリーズ at 39. Suspect X was a 33.
Thanks for the links. Yes too difficult for me I just started learning N3.
I’m currently reading 首切りサイクル by Nisio Isin and really enjoying it. It’s on the longer side and it was fairly difficult at first, but after the first hundred pages or so it’s been such a page turner that I can’t put it down. After getting used to the vocabulary and the distinctive way in which the narrator and some characters talk it’s gone fairly smoothly. It’s a locked-room mystery with Agatha Christie vibes.
If it seems to be a bit much, Nisio Isin has many other well-regarded mystery novels that are a bit more digestible. I definitely intend to read more sometime down the line.
Also, the upcoming Intermediate Bookclub Pick 地球星人 is not a mystery novel per se, but it is definitely an edge-of-the-seat page turner that uses relatively straightforward language, and you’d have the support of a bookclub.
Which book would that be? I checked a random book from the 三毛猫ホームズシリーズ (most of which are in the all you can read programme), and when I opened the book’s page and clicked on the picture, I got a 10-minute preview (you can read what you like, but I was asked to log in). Or was it a different book you were talking about?
Oh, thanks, I didn’t know that. I saw that I needed to log in and I thought I was being taken to join the all you can read. Good to know, not that 10 minutes would have been enough.
Thanks for the recommendation, it looks really interesting. Are there fantasy elements or is it a straight mystery?
I will most probably join that club when the time comes. I’ve read the first few pages of コンビニ人間 and really liked the writing. I thought I’d read that first, but what with the upcoming book club, it can probably wait. Glad to know that it’s a page turner!
Short answer: No fantasy elements.
Longer potential mild spoiler-y answer: There’s a character that supposedly has extra-sensory abilities, but whether or not they’re real is questionable to both the reader and some of the other characters in the book. I don’t know yet if/when I’ll get a definite answer.
So I ended up getting 三毛猫ホームズの推理. Thank you @lkyvic for the recommendation! I only just started reading, but I really like it so far. I’m finding it a little harder than コーヒーが冷めないうちに (I’m sure it’s just me getting used to a new writing style), but a much more entertaining read. I will be sure to post more about it after I’ve made some progress. 
To everyone else who recommended books, a big thank you as well! I’ve put most of these books on my to-read list, and I’m looking forward to getting around to reading them when the time comes. Of course, new recommendations are always welcome too. It’s always nice to have lots of great reading options to look forward to. 
Great to know, thanks! I am now more motivated to study I have been a bit slacking recently haha