I follow my friends, and my friends tend to have good taste! This is essentially my strategy. I do have a few friends who have recommended books that I didn't like, so I just don't listen to their recs anymore. Solved problem!
That's one of the problems I have with ML-based recommenders. I can't identify any particular ML algorithm, and therefore I can't say "This one has consistently given me bad recommendations, so I'll ignore it. But I do like this one over here." Whereas with my friends, I know that Lori and Nicole gives good recs and Brad doesn't, so I'll ignore Brad's and read recs from the other two.
If I can find similar tastes, maybe a little bit different, then that will result in good recommendations.
So I would want to find people who both like similar things but also dislike similar things. It will be interesting to find people my similarity and see which are best at finding books I like.