> The Comet Seekers by Helen Sedgwick — had never heard of this and would have never picked it up because of the low Goodreads rating

OK, some advice, and that is for goodreads, or amazon, or any place really. Unless you are really sure your taste matches exactly the average, mode, taste of the people rating in a certain place do not include or exclude books because of average rating. Take a look at the high ratings and low ratings, see which reviews strike you as more you.

Great you got great recs.

> It'll be interesting to me as I keep reading recommendations to see if I start encountering books that I think are just bad or whether the algorithm really can tell what "good for me" means!

Not just the algorithm, but the data the algorithm uses, which depends on the quality of questionnaire, but also on the quality of the reviewers which rated those metrics. I am somewhat dubious it will be reliable forever but I hope it keeps working for everybody who is happy with it.

>I don't give out 5 star ratings willy nilly.

Me neither, I understood. I did not like the Sparrow much (Mary Sue is kind of my main memory) and I would not get into the comet seekers. I would not have given you good recs! ( Though wild guess, if you do like sf, maybe you would like Connie Willis more serious books, maybe... though her only story about alien contact is a kind of a Christmas romp and very different in tone from those books).