As will become clear, I'm no kind of expert on the subject. If you want to read Japanese mysteries, you've probably tried searching the internet for information and suggestions about what to read. If you're like me, you've probably not found very much. Searching in English (or French or German) turns up a huge amount of useless pages and very little that is really about the subject. I can't search in Japanese. Although I can read Japanese on paper well enough, I find Japanese websites very trying, except for a few whose format I've slowly got to understand. But for what it's worth I thought I'd note down here the things I'd found while hunting round on the internet. (My interests tend more towards "puzzle" detective stories. So my searches probably missed hundreds of useful pages on tough guy gangster melodramas and other parts of the wider genre.)
There don't seem to be many English language blogs about books in Japanese at all. For detective stories, I've only found three, of which two are dead or dormant. The only one being updated at the moment is "The Case Files of Ho-Ling (浩寧の事件簿)", which is also the most wide ranging. It's mostly reviews of Japanese detective fiction, with some posts on detective games, detective manga, films and European or American detective stories. The emphasis is on puzzle detective stories. (There's also a page with secondary literature.)
The two inactive blogs are On the Threshold of Chaos (混沌の狭間), which has mostly reviews of Japanese detective fiction, with some European or American books, and How to escape a locked room (密室脱出方法) with a few reviews of Japanese detective fiction.
One thing I notice in all these blogs, is that hardly any of the writers discussed are women. Actually that applies to the detective stories I've written about here too. (I've read one book, a spy story, by NATSUKI Shizuko, but I wasn't planning to write about it.) There are certainly female mystery writers in Japan; but perhaps the puzzle oriented mystery has more male writers. If so, that would be quite a difference to Britain, where women probably make up the majority in the top ranks. I'll make an effort in the future to balance things out a bit in my choice of books.
As a rule, if a writer has had a book translated into English, they are likely to have a Wikipedia page, otherwise very unlikely, however famous they may be in Japan, e. g. HIGASHINO Keigo, SHIMADA Souji, MATSUMOTO Seichou. Wikipedia has a page on Japanese detective fiction, but it's really not very good. On the other hand, its "See also" section links to lists of winners of various Japanese mystery prizes, which seem well maintained and might suggest good books. (Sadly, at least one translation of prize lists from the Japanese Wikipedia have fallen to the English Wikipedia's deletion policy. So you may find it worthwhile hunting around there too.)
[Update: the text of the "Japanese detective fiction" page is still no good; but someone has added some very useful lists to the article since I wrote the paragraph above. Of course with Wikipedia, you never know it'll last.]
There's a page listing a desultory selection of impossible crimes here.
There's a page at Columbia by Satoru Saito with a bibliography of works on various popular genres, including mysteries, here.
Buying Japanese books can be expensive or difficult in Europe. One good supplement is to read older public domain works. Japan has relatively sane copyright duration,
fifty years after the author's death, which is at least better than the
seventy that Europe and many other countries currently have. That
means that many prewar writers and a few postwar writers who died young
are out of copyright (EDOGAWA Ranpo has a few years still to go; he died
in 1965.) Many public domain texts can be downloaded from Aozora Bunko (like Project Gutenberg but Japanese). I've only read a few short detective stories from this period, including three by OOSAKA Keikichi (大阪 圭吉). I mean to put up a translation of one of his stories here soon; but I need time to get the phrasing right and solve a few difficulties (or give up on them and guess).
I would really like to be able to read a good history of Japanese mysteries aimed more at the general reader than at an academic audience, something like Julian Symons' Bloody Murder. English language works are usually academic, which means that they are generally not good guides to the literature. Apart from the misrepresentation, there is the problem of spoilers. German has a booklet that seems aimed at serving as an introduction to Germans looking for "Krimis"; it doesn't read like the product of deep research and the number of writers covered is small. But it should certainly help a new (German speaking) reader looking for suggestions: Wittkamp, Robert, Mord in Japan. Der japanische Krimi und seine Helden: Vom Zweiten Weltkrieg bis zur Gegenwart, 2002 • ISBN 978-3-89129-745-2 · 132 pages. · EUR 12 (Eine Publikation der OAG Deutsche Gesellschaft für Natur- und Völkerkunde Ostasiens, Tokyo, im IUDICIUM Verlag). I'll try and look at some of the Japanese language guides to detective fiction in the near future. (Feel free to recommend one, if you know of a particularly good one.)