The majority of the books I read are books I have gotten for free. And as the title states, it's done legally so I'm not stealing anything from publishers or authors.
Aside from the obvious Library option, there are many sites that offer legal ways to get free books. The "surge" of free books started with NetGalley, the place that most people who blog know about, and probably use already.
After being on NetGalley and reviewing books, I started being asked by one author, in particular, to be on the mailing list for her when her books were coming out. I know I wasn't the only one.
I also got "invited" to be part of mailing lists for various mailing lists for publishers to know when new books were coming out and have the chance to request review copies. Some of my favorite books from the past two years are from these mailing lists.
I am not sure how I found this last one here. I haven't seen any books yet that interest me, but it's worth checking out anyways.