There is pretty much one scenario in which I will buy a book without reading it first, and that is if I go to a bookstore to have a browse and it catches my eye. (And I usually end up reading the first bit standing in the store, so if it's hooked me I'll buy it.)

Otherwise, I buy books if I already love the author (though this doesn't happen often aside from cookbooks) or if I borrowed it and loved it. I have a "love it enough to buy it" tag in Goodreads.