I often choose which book I'm going to read on recommendations or familiarity with the author's other works. Sometimes I'll pick a book with which I am familiar with other books in the line or series. Of course the reverse also holds true. If I read something by an author that I seriously disliked I will avoid all books by tha author in the future.

If I know nothing about an author or series, I'll usually go by a combination of cover art, title, and blurb. Sometimes it will be by pure random chance that I choose a book. If I need a book with a main character from Indiana or a setting that takes place in Michigan to satisfy one of the challenge rules for a reading group to which I belong, I just stop by the local library and say, "I'll take the first book you find with 'situation X'" and that will be what I read next. Or maybe I go to the library with my kids and say, "I'll get the first acceptable book with a car on the cover. The sooner I get a book the sooner we leave." And my boys being 15 and 17 know what an acceptable book is (not a reference manual and not a historical romance).