Go to Goodreads/Librarything, choose a subject/genre you never read and choose the 6th highest rated book (to avoid spam/flash popularity problems)
Personally I'm pretty doubtful this method will give you something you actually want to read though.
I've read about 20 books per year for the last three years. I buy most of them through Amazon and always mark them as read on Goodreads in order to get better recommendations.
And yet, I can't remember a single book that I have read exclusively based on showing up as recommended on Goodreads/Amazon. It's usually a word of mouth, some researching, and a couple of classics I've never gotten around to from time to time (currently finishing Fahrenheit 451 for example).
At best, the recommendations end up on my to-read list, usually forgotten, and disappear from there too when I do a cleaning of my to-read list because it got cluttered.
I assume it depends on the genres, especially with something like Goodreads. I mainly only read a single genre so it must be fairly easy for them to recommend me books.
To make the matter even more complicated, I sometimes read books from my own region, not translated into English (which, of course, makes them unavailable on Amazon), so the algorithm would have to count those ones into recommendations too.
I don't think that anyone is investing a lot of effort in solving this gigantic problem. Amazon just wants to sell you more books, it doesn't matter if you read them or not. I don't think that Goodreads has enough of the capacity to make their recommendations actually work for someone as complex as me.
I mean, if you generated one for each class of people, you'd probably find that there are easier statistical trends, but conceptually.
In fact, now that I think about it a bit more, you likely could use something like the Netflix algorithm and very broadly read people, who overlap on what you have read, then look for books they've read that score lowest on similarity to things you've read based on inverting the scoring mechanism.
It wouldn't be perfect, but with some tuning you could probably make that work as a simpler model than the DNN one.