For me, it sort of depends on the medium, if I’m reading an actual physical book, I go with authors and genres I like.
If reading on an electronic format, I will at least take a look at anything free.
Tinopus mentioned the free gutenburg project. I’ve used that one before.
Baen also has a free library that I’ve mostly exhausted at this point (doesn’t seem to change much). From there I do exactly what the Baen Free Library was meant to do, if I like a story, I go buy more of that author’s work to read.
There are tons of budding authors online who publish their work in blog format who are imho pretty good, plus lots and lots of free online libraries where I just randomly click titles till I find one that sucks me in.
I put essentially all historical mysteries I see in my to-read list on goodreads.
For all other genres, I tend to go by things like goodread recommendations based on books I’ve rated highly. I create shelves on goodreads to get specific suggested titles based on my own parameters.
Sometimes, I pay attention to what specific friends are reading, because their tastes are similar to mine.
Then what I read next usually involves some combination of what is available at the library for kindle and what I’m in the mood for.
I used to participate in the reading threads here, but have been lazy, lately.
I read a lot of books by independent authors nowadays, and I usually get recommendations from friends, Facebook acquaintances or just browsing Amazon for e-books.
I’ll troll through my library’s e-book non-fiction list once a week and get on the waiting list for anything that looks vaguely interesting. I usually have 3-4 library e-books checked out that I will probably skim through, but may read fully if they grab me. There may be anything.
I keep an Amazon wish list of books that have come up on blogs I read, NYT recommendations or other trusted sources. Mostly these are pretty wonky non-fiction books. Sometimes I’ll go look up “best books of the year” type lists and add the interesting ones to my list. When I’m feeling flush or my library selection isn’t inspiring me, I’ll pick up one of these.
About 3-4 times a year, I’ll pick a topic and get 5-6 books on it. Often it’s a place I’m traveling to or some work-related reading. In this case I’ll pick a mix of history, memoirs, travelogues, anthropology and fictions. I love being able to dive in to a subject from all angles.