For books that I enjoy I often read the reviews on Amazon (or the like) to see what others thought of the book. Where someone has written a thoughtful or well reasoned review I will read some of their other reviews as well. This has enabled me to discover books by new authors e.g., Jim Butcher where I have purchased whole series.
For your point about "loaned" books, Baen is the obvious example of a publisher who gives away early books by authors (which can be freely copied, as long as you don't charge for them) in order to generate sales of later books.
Whilst not exactly serendipitous examples they do help me discover books and authors I would (possibly) not have otherwise read.
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