It's not really possible in English to avoid mentioning whether you're talking about one thing or more than one thing. If you say "some bestseller," it means just one. If you say "some bestsellers," it means more than one. There isn't a way to say that you're reading "either one or more than one."
"Some bestsellers" sounds better, because "some" just means several. "Some bestseller" sounds a bit odd, as though you don't actually know what bestseller you're reading, or you know but you don't care about what book it is (you just picked it up randomly).