Yeah, I have had this issue as well. I usually start with "staff picks", but they aren't always that good. I suppose they choose a variety of books that highlight what all you can do with Blurb, as opposed to great books.
I have tried searching by key terms, but that necessitates that the author saw fit to keyword their book when they posted it on Blurb, which isn't always the case. I didn't do so at first when I made my books on Blurb, and only later went back and added key words.
So really, I've got nothing. But if someone figures something out I'd love to hear it.
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"Only a Sith deals in absolutes." - Obi-Wan Kenobi