I get the sense that part of the problem with book recs vs. YouTube video recs is that you can evaluate the quality of a video recommendation super quickly (click in, find out like 10 seconds in that you don't care for it, click out).

With books, actioning a recommendation involves

1) getting a copy of the book (digital or physical, both cost money or at minimum the time required to pirate),

2) starting to read it (with the sunken cost of time and/or money + the drive to "give it a chance" both looming over your shoulder, costing you more time),

3) eventually either getting to the end or swallowing your pride and bailing.

I think you'd start to see actually useful systems here if you could even eliminate step 1; make it easy as clicking in from a recommendation directly into reading mode.

Also solves a critical issue for the engine: gauge recommendation quality by how soon people click out of the rec, as opposed to waiting for the user to go through steps 1-3, and care enough to come back and provide a rating. Way more data to work with.

Of course, this all skirts the Actual Problem: the amount of IP law you'd have to trudge through in making enough books this accessible.