I don’t actively search for new things to read anymore, but here’s where I usually find new things to read (basically in order from most likely to find books to add to least likely).
When I find myself adding new stuff to my to read list, it’s usually thanks to other readers posting their reviews and recommendations on the WaniKani forums (specifically the Read Every Day Challenge Thread(s), Extensive Reading Thread, and Listening Challenge Thread. It helps to be familiar with what other people like to read because you can watch out for their posts specifically and look up books they mention.
Of course, Natively is quickly becoming one of those spaces where members can get recommendations from each other, so I often find intriguing books while watching the global activity. I admit I’ve added a lot of mystery books recently thanks to the new book club here as well.
One of the perks of living in Japan is having access to the local public library and even the school libraries (at the schools I teach). You can also see many recommendations and popular books displayed on shelves, which makes it easier to check out a book and see if it’s something interesting. The school librarian at my junior high school also creates a list of some of the students’ recommendations and circulates them among the staff and the students, so I’ve checked that as well.
I’ve only went to Book-Off to search for books in Japanese once (not including the times I went for picture books for my daughter), and I remember seeing certain popular books displayed on the shelves as well (with some review highlights). I added a few books to my to read list as a result.
When I go to check out books on Amazon (the synopsis, price, etc.) I’ll often check the other recommendations they provide based on what other books people check who also read/buy the book I’m looking at. I’ve added quite a few books from that.
The Bookwalker thread also on WaniKani really helps to remind me to check which books on Bookwalker are free to read (for a short period or to add to your bookshelf forever). But it doesn’t really get me looking at new titles I don’t know. I usually look for titles I do know and want to read (just for free, lol) so it doesn’t necessarily give me absolutely new material I didn’t already know exists. It just gives me another place to read stuff for free.
Line Manga is an app that you need to be in Japan to use, but it also has lists to help you explore more titles. Again, I don’t really look for new titles (it’s mostly manga for one, and I don’t like to pay for manga so I try not to read new titles so I don’t get invested in a series).
Personally I think it’s more useful than Bookwalker (both are free services but reading books on the Line Manga app is a lot faster than getting the pages to load on Bookwalker) but not as easy to get as many manga as possible to read for free (most are time limited too). But if you check often for special coin promotions, it’s easy to accumulate enough to save enough to buy manga or at least get a few chapters read (but they’ll only be available for 3 days).