Most books I read come from personal recommendations, a friend tells me they liked a book and think I will like it. If I read it and like it I might read others from the author.

Free books (and on sale <$1.00) are also an incentive and have more often than not resulted in me buying other books from the author.

Serendipity in book stores - not very frequent because bookstores are not close to me. I recently bought 3 books from various bookstores while looking at what was available. One I had been meaning to read for years the other two were by authors I knew of but had never read.

Random Internet reference, I read Stanger in a Strange Land because of the SO answer "You don't grok vi" - I wanted to grok grok.

Travel inspired, I read some Hemingway because I was going to Spain.

edit: Online reviews are like social media comments, bring some popcorn. If I want to read criticism about a book, it will be from a more reputable source. But, that is not to say I won't read a book with bad reviews from a more reputable source. Their negative spin might make me more curious.