I saw a recent pie chart of how readers find ebooks. I can't seem to find a link. so I can't share it, but the upshot was that people rely mostly on recommendations on groups they belong to, then recommendations by family and friends. Those slices combined were well over 50% of the chart.
In a book store (remember those?) it's called hand selling. That's when a knowledgeable clerk says "Oh, you like that book? Well, let me tell you about this one!" Even without book stores, it remains the most powerful selling tool for a writer.
This isn't self-promotion. Hey, I'm a writer, I understand letting people know that you have a book out. But it becomes just so much background noise especially to those of us with a lot of writer contacts. I don't think I've ever read self-promotion SPAM and thought, "Hmm. I must read that." However, I have on occasion said, "I will never read anything this writer ever writes because she's pissing me off with her ten posts a day about her book." I've heard that from others too. There's obviously a fine line here, and it's different for everyone, but one or two announcements are plenty. After that, you're teetering on the annoying edge.
So how do you tap into this all important 50% plus marketing vein? Well... You have to write a really good book. You have to write something that gets a reader excited enough that they talk about it. You can't force them to talk. You can't SPAM them into talking. All you can do is write such a great book that a complete stranger will tell another person, "You have to read this."
That's the big secret.
Now you know how it works.