for me, recommendations from amazon/users also bought, genre groups on facebook, author self-promos on here and piracy.
yes, piracy. my shelves and kindle are filled with (legitimately purchased) books/authors that i discovered through pirate sites. i dl a book and if i read and like it, i'll start buying the authors books (often in print, if applicable) and following their careers. and, being the compulsive shopper i am, i'll often purchase the complete bibliography in one fell swoop due to my enjoyment of one 'ill gotten' book. theres a particular indie fantasy author who i ended up purchasing not only individual print books by but the print omnibuses as well, essentially buying them all twice, all off of one ill gotten download. and then i'll go on facebook or amazon to see who they enjoy or recommend and i'll start buying books from them too. one seed leads to 50 branches of me buying books.
of course the process is a lot less complex than my drivel would imply.
(and no i don't pirate regularly, a handful of times each month at most. i don't just take to take, i'm not greedy or abusive. i have so many purchased books to get to that i don't need a constant influx of ill gotten ones.)